Wizkids Rules!

If a Possession character is not blocked where does the die go? Normally unblocked characters go to the used pile. Would this mean the possessed character die would return to my opponent's used pile at the end of my turn?

From the rulebook under Capturing, Controlling, and Copying:

If the controlled die is sent to your Prep Area or Used Pile, it goes to your opponent’s Prep Area instead.

So as an unblocked character, it would try to go to the Used Pile, but instead goes to your opponent's Prep Area.

-The Dice Masters Rules Team