This is a question of identity, and how these things interact:
Say Assassin's Strike has dealt damage after using its own E. Would it still be placed in the staging area if Possession Stance turned it face down? I ask because I am unsure if the clause "this attack" carries over to when the ability would finish. Because the attack would be face down, it would no longer be considered an attack (or possibly even "this attack") when the End Phase happens.
Really just a clarification, but one that is bound to show up.
Second thing. Can a multiple be turned face down again? Do they turn face up?
-Tinman