Detect Possessed unit?

when a unit uses the possess ability, is there any way to detect the unit who is being possessed?

Simply use the "Finishes casting an ability" event for the trigger. To refer to the unit being being possessed, use "Target unit of ability being cast."

Hope it helps!

It doesn't, because as far as I remember that event doesn't have that event response. Use the event "starts the effect of an ability" instead. (if this event happens too early with possesion (I remembered now that the spell is channeled in some way) then just use this event to store the targeted unit into a variable and then use that variable with the "finishes casting" event; however, that can get complicated if you want multi-instancability (more units being able to use the spell at the sam time))