While the Gloomy Forest is a dense, old-growth woodland, it also lies in the highlands of Brackleborn province. The underlying terrain consists of steep-sided hills, with deep valleys running between them. Climbing the hills involves rigorous activity that eats up valuable daylight, meaning the party has tended to follow the maze of gullies, culverts, and channels that characterize the forest at ground level. They haven't quite realized how many areas and encounters are up on the hillsides that they've simply wandered beneath without ever realizing it.
Some of these encounter areas are lairs, the denizens of which populate the wandering encounter tables. If the players can locate a lair and wipe it out, then that creature type might disappear entirely from the tables. While a lair persists, it also generates an additional encounter check if the party wanders within a certain distance of it. Thus, wiping out a lair of monsters can have long-reaching exploration benefits.
I've sprinkled lots of these encounter lairs across the Gloomy Forest map: peryton nests; an awakened grove full of intelligent animals, plants, and myconids; a bullywug village; an ancient cemetery; an ettercap nest; etc.
One such lair is the following owlbear den, a bit of content I cribbed from my old Frozen North campaign. With a few minor flavor changes from a Viking-style setting to a European/fey one, the content works just as well here. Reusing adventures is something I'd always avoided in the past...no idea why. It saves so much time and effort, and let's me revisit the adventure with a fresh perspective. With the right changes, I've even reused adventures for the original players and they didn't catch on.
As with most monsters in the 5e Monster Manual (particularly the classic monsters), I've had to de-neuter their archetypal special attacks. I don't know why 5e took away some of the core monster abilities (like the owlbear’s classic hug attack, or the rust monster’s ability to eat magic weapons and armor...it just makes no sense). I digress; my owlbears will hug the crap out of you. Changes to the core monster are described below.
Owlbear Den
This is the lair of a sleuth of owlbears—(8) “sows,” (6) juveniles, and a massive “boar.”