When I was bringing my first 5e campaign to a close, the PCs were trying to rid themselves of a curse they had acquired after finding and removing an evil idol from a warded vault deep beneath the earth (intending to have it destroyed). The idol depicted a tentacled, demonic figure that they could not rid themselves of—no matter what they did or where they tried to leave it, it always reappeared among their belongings.
Part of the curse revealed the party's location to sinister beings from beyond (mind flayers) who sent their minions to attempt to retrieve it. These minions included gnolls, frog-demons (slaad), and members of the Cult of Khoss. The cult misguidedly thought they could subvert the mind flayers to their cause, but soon found themselves under the thrall of the alien geniuses.
A faction of the cultists who remained free of the mind flayers' control made contact with the PCs and divulged that the mind flayers' base of operations (and the source of the idol's cursed power) lay within the Howling Hills of northwestern Remedios, among the ruins of an ancient civilization that once worshipped the mind flayers. The cultists provided the party with another piece of crucial information: The mind flayers already possessed two similar idols, but needed the party's third idol to open a portal to their beyond-realm, through which "something" would emerge to conquer this world.
Finally, the cultists gave the party directions to the site. The cultists' plan was to convince the party to go to the temple and do the dirty work of wiping out the mind flayers (or at least distracting them) while they freed their enthralled comrades and escaped the area. Whether the party succeeded or not was irrelevant to the cult (Glorious chaos!); they simply wanted to extract themselves from a situation that had gotten out of their control. This was the background for the campaign's grand finale.