The Hurricane Dungeon – The Chambers of Woe (Level 6)

The Level 6 map is actually the last of the series, the ninth map I drew post-hurricane (and, mercifully, our final night without power, which came on the next afternoon). This is also the last of the "finished" levels in which there is stonework, excavated rooms and corridors, and features like doors and such. The remaining three levels are caves and caverns (with the exception of a finished temple area on Level 9).

I recall designing this map specifically as a maze with chambers, rather than a functional "place." With a few exceptions, the rooms are 30x30 with doors at one or more cardinal points, and most corridors are 50' or less in length. It's a tight, circuitous space without obvious purpose. It reminds me of an architectural circuit board, which perhaps has some sacred geometric significance for the empress's arcane machinations.

 

Compared to other levels, there are fewer secret doors, but all the larger rooms are hidden behind them. I don't recall making that deliberate creative choice, but it's an interesting one from a design point of view. It will also create these tantalizing blank spaces on the party's map, obvious secret locations with difficult to locate entrances far removed from the actual area. I see this level as a place where the empress hid important things inside a baffling maze. (She's insane, so who knows what she's thinking?)

There are also four staircases to other levels, one ascending and three descending. I was able to incorporate the ascending stair into the map for Level 5, and I had two additional staircases down from Level 5 that I needed to add to this map. Finally, I rolled four access routes on the stocking rolls for this level: A chute down one level, a stair down 2 levels, a stair down 3 levels,  and a chimney down 2 levels. Finally-finally, the original map has two teleporting hallways which could be interconnected, or they could lead to other areas/levels.

In the end, trying to add more multi-level stairs into the cavern levels below proved too tricky to resolve in a way that was satisfying, so I kept the chute and the chimney routes, and counted the two teleporting hallways as the stocked stairs, one of which goes to Level 8 and the other to Level 9 (as a backdoor into the evil temple). Doing so also provided me with a rationale for the fire giant who lives on this level. With shading, titles, and all the incidental features added, the map ended up like this:

 My content stocking rolls (for a 44-room dungeon level) came up with the following results:

–Empty: 16 (36% / 35%)
–Monster: 8 (18% / 20%)
–Monster + Treasure: 10 (23% / 25%)
–Stairs: 4 (9% / 5%)
–Trick/Trap: 3 (7% / 5%)
–Treasure: 3 (7% / 5%)

The first % number is the percentage of the actual content rolls; the second % number is the typical expected percentage spread (per OSRIC guidelines).

Ophidian Temple (2025 Adventure Site Contest Submission)

Taking a brief break from writing up level 6 of The Hurricane Dungeon to talk about this year's Adventure Site Contest hosted by Coldlight Press. I'm not judging this year (I may have "overstayed my welcome" with such long and in-depth breakdowns of each submission), but I am definitely entering. My submission for the first contest, Etta Capp's Cottage, won a spot in the final publication. Last year's entry, Owlbear Hill, was a bit too ambitious and failed to make the cut.

I just turned in my submission for the third annual contest, though I actually finished it back in August (the contest didn't open until November 1). I've titled this one Ophidian Temple (not great, but precise... I just couldn't land on a better title that didn't sound dumb or trite). It's written for AD&D and intended for 4–6 player characters of 5th to 7th level. The elevator pitch for the adventure reads:

Drums in the jungle herald the return of the snake-men to their evil temple.
Blood sacrifices to their demon-god begin anew.
Who will stand against their evil plans?

The site began as a sketch map I doodled in one of my notebooks many years ago (probably early-2000s). Over the summer, I pulled my box of old, hand-written/drawn game materials out of the attic and scanned everything in: Unfinished adventures, house rules ideas, character sheets and sketches, etc. Forty years worth (sheesh!). This map was in that lot. It's pretty simple and has no liner notes or indications of what it was intended to be. I doubt it was anything more than a random scribble while I was working on something else. It had a pleasing shape though, so I decided to put it to use after many years. I love it that an idea from the past has found a home in the future.

The original map had a few problems I needed to address to make it an adventure worthy of a Classic Adventure Gaming title. For one, it's pretty linear despite the side branches; not very suitable for CAG-style exploration. The layout also creates several bottlenecks for both player characters and enemies that had the potential to make dungeon combat a series of hallway fights between opposing sides of 2–3 combatants. That happens frequently in D&D, and smart players know how to use bottlenecks to funnel large numbers of opponents into a manageable front line, but I wanted to open this one up a bit.

The layout and details I drew suggested a tomb structure of some kind, nestled in the back of a canyon or maybe a deep cave. There's also a throne room, which isn't necessarily out of place. Tombs are a bit limited in terms of monster selection, though, mostly undead, vermin, constructs like living statues and such. As I thought about what I would want to say with yet another tomb dungeon, I came up blank. More accurately, I couldn't find anything interesting to do with it that I (and others) haven't done a million times.

In my Keep on the Badlands sandbox campaign, I had a whole adventure area populated by yuan-ti: A hidden valley filled with crumbling temples, dinosaurs, a giant ape, and terraced gardens subsumed into the rain forest. Unfortunately, my players never quite got to the yuan-ti area and their temple went largely undeveloped (the players gave up two-thirds of the way into the valley).

The idea of the yuan-ti temple stuck with me, though. Yes, it's still trope-y as hell, but I've never done much with this type of setting other than when I ran my version of Dwellers of the Forbidden City back in the 80s. This, despite my love of Tarzan novels, movies about jungle exploration and lost cities, and tales of dinosaurs and other monstrous versions of real-world creatures. A formative influence was watching Land of the Lost on Saturday mornings as a kid. The world-building in that show is incredible and would make for a banging campaign area.

 

With a yuan-ti temple in mind, I dropped the original map scan into Photoshop, added a grid, and then sketched out the contours of the dungeon, fixing some of the issues to make the site more interesting. I also dropped in a few notes about the dungeon contents.

 

This is almost identical to the final version I used.

As I started reading through the monster descriptions to sort out how I would place and use each encounter, I really fell out of love with the yuan-ti for this site. I never much liked them to begin with; they're not terrible, but they have a high degree of specificity and a fair amount of complexity, which is not something you want in an adventure site with a hard page limit. I planned on using the ophidians from Monster Manual II as minions of the yuan-ti, but then I realized they work perfectly fine as the primary antagonists.

I never paid ophidians much attention before now. My previous 20+ year campaign was firmly rooted in a classic, Northern European/Scandinavian-style fantasy setting, so snake-men were far from my typical antagonists. My 5e Badlands campaign was set in a Southwestern US/Meso-American-style desert region with adjacent jungles, which was perfect for using monsters I'd never really employed before. Even then, I overlooked the ophidians.

I think this has a lot to do with two things: One, the MMII is not very good. There are some standout creatures for sure (though the best of these were introduced in earlier modules), but most are either meh or borderline stupid, or just variations on another monster (the Fiend Folio is full of this as well). The second problem is the artwork. It's boring.

There are four artists listed: Jim Holloway, Harry Quinn, Dave Sutherland, and Larry Elmore (who did the awful cover). I honestly don't like Elmore's art style, and the way that ogrish-looking creature wields his halberd is completely backward. I attribute Elmore's art with the decline of AD&D (contemporaneously, not causally) and so it triggers a certain revulsion in me, perhaps unfairly. Same with Harry Quinn. His stuff is just ugly and uninspiring. I didn't care much for DCS's art back in the day, although I pored over every inch of his illustrations, but now I have a great deal of respect for his giant-sized contribution to the look of the game (and modern fantasy, for that matter). His work in the MMII is not his best, however.

Jim Holloway accounts for maybe half of the illustrations. I love his character stuff and he has a great eye for setting up a (usually comedic) scene that still looks like classic D&D, ridiculous situations that you could see happening to player characters. His creature designs, on the other hand, leave a lot to be desired. They're not bad, per se; just bland and static, lacking any of the dyamism or creativity in his character illustrations. Behold: The Ophidian. *Yawn.*

He looks like he just got out of bed. Yeah, it's a snake with arms, big deal. It's so dull, your eyes sort of wander over it and then move on. He should be coiled upright, weapon and shield raised, baring his fangs dripping with poison, setting his beady, soulless eyes on his prey. Instead, we get this and it sucks. I imagine these guys were rushed by the production deadline, underpaid (or not paid), and just DGAF because the company was falling apart around them. Still, what might have been otherwise remains a tantalizing dream.

In any case, the ophidian! It's not bad at all. They're natural minions with 3–4 hit dice, so in the same upper class as creatures like jackalweres, wererats, and ogres. They have natural armor the equivalent of mail, and can use weapons and shields to boost their combat numbers. Their bite attack isn't that great, but an extra 1–3 damage attack is nothing to dismiss, either. Their venom inflicts a lycanthropy-type disease that slowly turns the victim into an ophidian over the course of 2–3 weeks. (It was this mutability that gave me the idea for the big boss: A snake-ape hybrid demon.) The disease is easily curable though, so unless it goes untreated, it's not too big a long-term threat (and a non-existent one if the adventure is played only as a one-shot). Still, it's a neat idea that could spin into all sorts of complications for the party in a long-form campaign.

That's basically it: Snakes with weapons. Perfectly simple for what I want to do here. Moving on, when I think of jungles, I always think of giant ants, so they're in as well. D&D giant ants are no joke. I talk about this in another blog post for the Hurricane Dungeon. The 3 HD giant soldier ants have a poison sting that is poorly written in the Monster Manual and completely mistranslated in OSRIC, but even the 2 HD workers can be nasty in numbers. An unfortunate wandering encounter of just two worker ants chewed through a 1st- and 2nd-level party in my Sinister Secret of Zenopus' Tower campaign, resulting in a near-TPK.

I also wanted a plant monster of some kind that wasn't a yellow musk creeper or shambling mound (both of which are present in the Hurricane Dungeon levels). There are the weird flowers and the wolf-in-sheep's-clothing from S3 in the MMII, but I remembered there being a couple of other plant creatures as well. While flipping through the book, I came across the mandragora (which I am committed to using somewhere as it's kinda neat) and the mantrap just below it, which turned out to be precisely what I wanted.

Using plant creatures in subterranean settings is always challenging for a designer without resorting to a hand-wavey magical solution. In this case, I created a collapse in the ceiling through which the jungle has entered the complex, allowing for an organic (and logical) scenario to unfold. Another recurring problem with plant monsters is their lack of mobility, making them easy targets for missile fire and spell casting, so I threw in a few giant boring beetles to make this encounter harder, tactically. Boring beetles are most certainly not boring, however; they are jaw-droppingly deadly with 5 HD, plate armor carapace, and a mandible attack that does frikkin 5–20 damage. One of these nearly demolished our party of 4th-to-6th-level characters in Prince's campaign.

They are also an anomaly. Of all the giant beetle species, they are the only ones that include any treasure type at all, and theirs is a doozy! A combined C, R, S, T on the treasure tables is comparable to a low-grade dragon's hoard, with mid-range chances for lots of gold, platinum, gems, jewelry, potions, and scrolls, and modest chances for copper, silver, and electrum coins and a couple of magic items to boot. Our group in Prince's campaign stumbled across a beetle in its lair and made out like bandits. The beetle's description doesn't say why this is, though it mentions a communal intelligence in some cases. My theory is that this is a misplaced treasure type, a publishing error that belongs somewhere else. The only creature with a similar type is the triton (C, R, S, T, X).

I picked two other monsters to reinforce the serpentine theme of the place. The necrophidius, or "death worm," is an interesting creature. It looks like a human skull with fangs on top of a skeletal snake's body, but it's not an undead; it's a golem-like construct created to serve as either assassins or guardians. I've used them on several occasions and, while they might not be that tough physically, they are quite deadly. They have a hypnotizing effect that renders victims who fail their save helpless. On top of that, their venomous bite paralyzes, which also renders victims helpless. It doesn't say this in the monster's text description, but I added a reminder in the adventure key that their bite against a helpless victim, according to the rules, would be equivalent to an assassination check on the assassin's table. This creature is perfectly capable of one-shot killing any of its victims. That's nasty.

The other choice was a couatl. I've always enjoyed putting little places within an adventure in which the party can get some sort of boost or helpful aid. I hid the couatl in a secret treasure vault and trapped it inside a "good" version of an iron flask. That seems like the sort of prisoner an evil snake-man cult would keep. If the party frees her, she can cast some healing/recovery spells for them. I left open the possibility that a Lawful Good character could persuade her to help their fight, but they would have to be of the Lawfully-goodest type and their current situation would need to be dire for me (the couatl) to intervene like that.

Finally, I created the main antagonists: A snake-man priestess, who is mostly just a human cleric with snake-like features including scaly skin (natural AC) and poison fangs, and the snake-ape demon "god" who isn't present unless the party dicks around and allows the priestess to complete her summoning ritual.

Speaking of that, I included several environmental effects in the temple as well. One of these is the ever-present sound of frantic drumming and droning chants by the cultists as they work to bring forth their demonic master. While the party is present, the pace and intensity of the drumming/chanting increases, signalling to the PCs that they better get a move on. The players won't know it exactly, but they have 24 turns (4 hours) to put an end to this or the snake-ape demon arrives in the main temple and will add to the difficulty of the final fight.

Another effect is that the temple is filled with clouds of herbal incense being burned in copious amounts in the summoning areas. It smells unpleasant but isn't harmful in the outer areas. As the party gets closer to the central chamber, however, the smoke's narcotic effects can overcome them after just ten minutes. This will require the party to either figure out a way to remove the smoke, or beat down the cultists in less than 10 rounds. Otherwise, poison saves to avoid falling unconscious are on the menu. Anyone who stays unconscious in the smoke will eventually die of an overdose. The smoke stimulates the snake-men but is not toxic to them.

A third effect is the pollen of the mantrap plant. It is so fragrant and pleasing that it nullifies the smell of incense in the chamber, and has a hypnotizing effect as well. Victims who fail their saves are compelled to approach the main plant, which then envelops and digests them in its leafy fronds. Lovely.

I placed several traps also: A spear trap in a central hallway and paralyzing darts from the walls in another chamber. Both traps are triggered by stepping on certain floor tiles, and the snake-men know which ones to avoid. The third trap is in the necrophidius' lair, and its a simple tripwire that closes and locks the entrance door, to split the party and allow the necrophidius to more easily kill its isolated prey.

I don't recall how I selected the treasure. After so many random stocking rolls made for the Hurricane Dungeon, it all kind of blends together. I'm certain I chose the Staff of the Python to reinforce the snake theme, and maybe gave the priestess the Cloak of Protection, but the others feel randomly rolled. Who knows? I was very generous with the money, however, though much of it is hidden and/or tricky to obtain.

Players who defeat all the monsters and find all the treasure can expect to net about 175–250,000 xp, depending on how much magic treasure they keep or sell. This is around 35–50k each for a party of 5 PCs, which is enough to go from 4th to 5th level, or 5th to 6th level, and a fair chunk of the way from 6th to 7th.

You can download the PDF here.

The Hurricane Dungeon – The Chambers of Woe (Level 5)

Continuing my series on creating this megadungeon, this is the seventh map I drew during the power outage, which became Level 5. Again, this is because I wanted to keep the "finished" levels and the cavern levels together (with the caves being the lowest levels). The original map feels a little uninspired compared to the others, though it is probably a more "classic" dungeon layout.

There are a few interesting features on closer look. Large rooms and long corridors, lots of loopey-doopiness, many secret doors and passageways, false doors marked with a 'T' for "traps," and hey, look at that...stairs to the levels above and below! It's not too bad, actually. The central, cross-shaped feature, includes directional arrows and four movement options. That's cool. (I was certain I cribbed this idea from somewhere, and sure enough, a similar feature is in the OD&D Sample Dungeon.) So, I felt pretty good about what I had to work with as I started fleshing out this level, even if it's a little Plain Jane.

I had already connected the staircase up to Level 4, and I lined up the stairs down to an area on the map I was using for Level 6. That map for Level 6 also contained a staircase up in a different location, so I had to figure out that connection as well. The stocking rolls for Level 4 had also indicated two stairs, a pit shaft, and a trap door and ladder leading to this level, but I had already placed those by the time I began work on Level 5. Finally, my stocking rolls for Level 5 included an additional two staircases and a chimney exiting down from here. Once I added those access routes, along with the room key numbers, a title, and shading, the level looks like this:

 My content stocking rolls (for a 35-room dungeon level) came up with the following results:

–Empty: 11 (31% / 35%)
–Monster: 4 (11.5% / 20%)
–Monster + Treasure: 11 (31% / 25%)
–Stairs: 3 (9% / 5%)
–Trick/Trap: 5 (14% / 5%)
–Treasure: 1 (3% / 5%)

The first % number is the percentage of the actual content rolls; the second % number is the typical expected percentage spread (per OSRIC guidelines).

The Hurricane Dungeon – The Chambers of Woe (Level 4)

The fourth level uses the sixth map I drew, this one on October 2, 2024. Maps 4 and 5 were both cave networks, but I wanted to keep all six "finished" dungeon levels together, with the three cavern levels being at the bottom so I moved the order around. This particular level had some interesting features: Large rooms, a chamber with a huge pool of some sort, and a massive central chamber with an altar and four columned wings projecting out from it. A few secret areas, lots of statues, and a second pool of water provided additional inspiration but, like the previous level map, this one included no access points, neither stairs nor passages leading off-map.

I knew from keying Level 3 that I had added three stair locations down to this level, and there was a fourth stair down from Level 2 as well. I also knew that the next map I was going to use for Level 5 had an access stair drawn on the original, so I would have to place that stairway down on this map. Finally, my stocking rolls came up with five (!) access points, more than twice the average statistical percentage for a level this size. I didn't end up using all of the stocking access points as rolled, which I'll get into further below. After adding all the access points, title, and location numbers, the map looks like this:

My content stocking rolls (for a 41-room dungeon level) came up with the following results:

–Empty: 11 (32% / 35%)
–Monster: 7 (14.5% / 20%)
–Monster + Treasure: 13 (29% / 25%)
–Stairs: 5 (12% / 5%)
–Trick/Trap: 2 (5% / 5%)
–Treasure: 3 (7% / 5%)

The first % number is the percentage of the actual content rolls; the second % number is the typical expected percentage spread (per OSRIC guidelines). Pretty close, statistically, with the "Stairs" result being the only real deviation.

The Hurricane Dungeon – The Upper Works (Level 3)

Level 3 was the third map I drew during the power outage. The direct correlation between creation date and level number ends with this one, however. The level has a few unique features to the other maps I'd drawn so far. For one: It has an entrance portico and an exterior area (bottom); two: It contains no routes (stairs, etc.) to other levels; and three: It has a teleporting passage similar to Level 1, but its destination is "off-screen."

Again, these maps were just unrelated doodles at the time I drew them so I wasn't concerned with logical structure or access points. This particular map's weird features (at least the first two) created some design challenges as I developed these maps into the mega-dungeon.

The problem posed by the front entrance/exterior comes when lining up the maps. I didn't want to "shift" any of the maps if I didn't have to. In this case, if you lay the first three maps on top of each other, the exterior cliff wall drawn on level 3 intersects with a few areas on levels 1 and 2 above it. It's a discrepancy that's fairly easy to hand-wave if you're not nitpickey. But I am, so I accounted for the exterior space in the key, describing it as a recessed area carved into the cliffside. This kinda deal: >>>

Not the greatest solution, but it works. Especially because shifting the dungeon would have complicated some of the level connections that I still needed to add to the map. There were already two stairs and an elevator to be added from Level 2, and my stocking rolls added another two stairs down to Level 4. Counting the main entrance and teleporting passage as additional routes, Level 3 has seven points of access – not bad at all. (I added an eighth access point in the stirges' room, and the dungeon's ventilation system adds even more, unquantified access points, but both require miniaturization of some sort so I'm not counting them in the total.)

After placing all the new stairs and marking the destination point of the elevator, and then adding a title and key, wall shading, and some incidental details (such as blockages and a pit trap), the map came out like this:

 My content stocking rolls (for a 37-room dungeon level) came up with the following results:

–Empty: 16 (43% / 35%)
–Monster: 3 (8% / 20%)
–Monster + Treasure: 14 (38% / 25%)
–Stairs: 2 (5% / 5%)
–Trick/Trap: 0 (0% / 5%)
–Treasure: 2 (5% / 5%)

The first % number is the percentage of the actual content rolls; the second % number is the typical expected percentage spread (per OSRIC guidelines).

The Hurricane Dungeon – The Upper Works (Level 2)

With the Level 1 key under my belt, writing the key for Level 2 was simply a matter of refining the format using the map I drew on our second night without power. Note that I did connect this map with the first map, via the stairway in the center-west portion of the map. I included only one other exit from the level to the east-southeast, hidden behind a secret door. 

There was also the nearby circular room with two levers and a floor circle, which I recall thinking of as some sort of elevator platform/shaft while drawing; but it, too, was concealed behind secret doors. Getting off this level would be a challenge, as providing those inter-level connections wasn't an objective at the time I was just doodling a map. I also wish I had drawn passages leading off the map, to expand the level horizontally. I mean, I still can with some Photoshop trickery, but I wish I'd thought to.

In designing Level 1 for this megadungeon, I added two stairs down to level 2, so I knew I had to add those connections to this map as well. My stocking rolls came up with two additional "Stair" results: one "Stair down 1 level" and one "Stair down 2 levels." The descending stair on the original map (near the "elevator" room) was proving difficult to sync up with the map for level 3, so I made that stair the one that went down two levels (to Level 4) and made both newly-rolled stairs a single-level drop to Level 3. The elevator platform ended up also descending one level to Level 3. 

After adding the new points of egress, keyed room numbers, a title, and shading, the map looked like this:

 My content stocking rolls (for a 40-room dungeon level) came up with the following results:

–Empty: 11 (27.5% / 35%)
–Monster: 10 (25% / 20%)
–Monster + Treasure: 13 (32.5% / 25%)
–Stairs: 2 (5% / 5%)
–Trick/Trap: 2 (5% / 5%)
–Treasure: 2 (5% / 5%)

The first % number is the percentage of the actual content rolls; the second % number is the typical expected percentage spread (per OSRIC guidelines).

Classic Monsters Revisited – The Tween (Fiend Folio)

While stocking the Hurricane Dungeon, one of the monster results came up as a tween from the AD&D Fiend Folio. That book is mostly a collection of monsters from the UK gaming magazine, White Dwarf, which covered a variety of RPGs in the early days before it became the house mag for Warhammer. Many of these creatures first appeared as submissions to the magazine's Fiend Factory column, by authors of varying levels of expertise and creativity.

Some are well-crafted and interesting, but most are simply variations on other monsters, with few distinguishing characteristics (other than appearance), or are just completely unnecessary. A handful of monsters were contributed by Gary Gygax, pulled from his previously-written adventures and added to what was essentially the second "monster manual" before the third monster manual became the official Monster Manual 2. Certain Fiend Folio monsters are passable-to-great; the rest are hit-or-miss (with lots of complete whiffs).

Having never used a tween before, I was eager to give it a fresh look. The tween's most interesting characteristic is a rudimentary form of 5e's Advantage/Disadvantage mechanic. (Perhaps even its "inspiration"?) The tween is essentially an ethereal "ghost" that attaches itself to a host, forming a shadowy doppelganger that provides a tremendous benefit to its host, but at a high cost to everyone else. The creature's presence is summed up with this wildly generous understatement: 

A character with a tween partner is therefore something of a mixed blessing to any companions.

That's because, while the tween's "partner" (it's host organism) gains the ability to make two dice rolls whenever a roll is required and take the better result, every other creature within fifty feet of the tween must do the same and take the worse result. This includes the tween host's allies. It's a "luck eater," basically, transferring the misfortune of others to the benefit of its host (and it, presumably). This looks awesome on the surface, but it has some enormous flaws.

 For one, by adding this to your campaign, you're doubling nearly every roll made at the table, on both the players' and the DM's side. The description of this ability reads:

(A)ny character or creature with a tween 'partner' has two die rolls instead of one, whenever a die roll is called for, and may select the more advantageous of these rolls. (This applies to 'to hit' rolls, saving throws and the like.)

The phrase, "...whenever a die roll is called for" is a bit ambiguous as to the type. (Is it all die rolls?) We can assume, based on the two examples provided, that the design intends for this to apply only to rolls made with a d20 (as is the case with Advantage in 5e), and not to rolls for things like hit points, weapon damage, etc.

But what about percentile rolls for success, such as for thief skills? The tween's description suggests that it is manipulating the environment around its host to make this "luck" happen (guiding the host's weapon, for example), so shouldn't it also apply to finding a trap or picking a lock? 

In any case, the result is a lot of extra rolling for the host and his fellow party members. And that's not even taking into account that the DM will have to roll twice for every single NPC and monster within 50' and run all those numbers as well. The game will inevitably slow to a dice-rolling, number-crunching crawl.

A second flaw with this design is that the tween's presence would be an intolerable situation for most players (I would think). I mean, if I were a player in this group, my character would have to deal with the situation in the short term until we left the dungeon and got back to base, but that tween-inflicted character won't be making a return trip (not with my guy, at least). 

I can't imagine any party of players accepting "bad luck" on every roll (according to statistical breakdowns of 5e Disadvantage, this equates to –5 or so on average). There's just no way. That's an enormous penalty in AD&D and a recipe for interpersonal conflict between characters (and perhaps even players). It will get someone killed eventually. I can see a character being forced out of a group over this situation, which sucks. It's one thing if the characters create that conflict themselves; it's quite another when a random roll forces it on them.

Worse still, the tween can't be countered by normal spell means, like Remove Curse. The description says that once a tween selects its host, it "...will remain permanently with him until he or the tween dies." That's pretty definitive, so it doesn't sound like the party can convince the creature to bugger off. The only way to be rid of the tween's luck effect is to enter the Ethereal Plane and kill it. But it's a Neutral creature ostensibly trying to help its host, which may pose a moral dilemma for Good characters (this is kind of cool). A lenient reading of one line gives DMs a bit of an out if they want to grasp it: 

The tween has the ability to see a few seconds into the future and is able to increase its host's luck.

Those two clauses suggest that perhaps this ability can be turned on and off by the tween, instead of being always-on. That would change the calculus dramatically, giving the player an incredible ability, but requiring them to use it situationally (mainly by separating himself from the party by 50' or more during combat... a tricky proposition). Still, a lone thief with such a tween would be a powerhouse. 

I don't believe that is the intent, though. I think this is just a dick-move monster, as designed. This is not an uncommon situation with the Fiend Folio.

I've kept the tween encounter in the Hurricane Dungeon to stay true to the dice rolls, and I'm curious to see what happens if one of my players ever "acquires" it. In the level 2 writeup, I adjusted the tween's effect to be a straight –4 / –20% penalty to others, so as to reduce the number of rolls the DM has to make. The host rolls twice for their bonus, and player allies can opt to either take the straight penalty or make two rolls at disadvantage instead.

For my home game, I may even alter the tween's effect even further to make it on/off (at will, triggered at the beginning of the round), and say that every time a player used the advantage ability, a single random creature — friend or foe — within 50' would suffer the disadvantage roll (instead of every other creature). 

That way, it's a bit more balanced and usable, and the number of extra rolls is kept to a minimum. I like magic items with caveats that make them somewhat dangerous to use, and this is like that. Fellow players may be able to more easily swallow the choice to accept a personal risk of penalty in order to give a comrade a big extra bonus in a critical moment (and potentially inflict the penalty on an enemy). It then becomes a tactical decision, rather than the "mixed blessing" of an arbitrary, likely-lethal, party-busting curse.