Quote Originally Posted by Maerok View Post
Screw with space and time.

Surround them. Isolate them.

Replace them with impostors until only one PC remains in the 'party' and even the other players, as doppelgangers, don't know it. Or, make them all impostors and see how long it lasts before they notice. Then the real 'party' kills them off and the adventure continues.
Have them fight themselves and then find out that they're the doppelgangers when they don't bleed or change back when dead.