Upon triggering, the GM has everyone roll dice, and writes down the result. Low number loses, but this is not shared immediately. The victim has been inflicted with creeping paranoia.
It takes time to manifest, but slowly, as the game session goes along, the GM passes the player notes (either overtly, in front of the other players, or in secret), implying that one of the other characters, then two, then maybe all of them appear to be doing something shady. In time, shady becomes sinister, and sinister becomes apparently hostile.
Managed correctly, the player will think everyone else has been affected by the curse, turning them all evil or dangerous. A successful Remove Curse, Dispel Magic, or appropriate Healing (GM's choice) brings the character back to normal. The curse ends on its own once the character leaves the dungeon.
Optional: The curse doesn't end on its own when the character leaves the dungeon.