There is an imposter among us... invasion of the body snatchers! A shapeshifter who can take on any form and wants to kill everyone and have you to themselves. This is very open scenario. Iden will want you. For example, among us, make them an alien, stuck on a space ship where people are getting killed off. Or isolated laboratory where the test subject escaped and start to kill the scientist. Or fantasy, you and your adverture party are trap in a dungeon or just escape a dungeon, but one of your party members seem different.