I'd go with a more recent classic from Age of Empires, the monk and its "wololo" ability to convert enemies to become your unit. To keep it balanced, maybe it should only be able to convert certain pieces and taking more than 2 turns to complete its conversion as to give the opponent a chance to counter its special move.
EDIT: Totally off topic,but..
I just found out that if you put an evoker in minecraft next to a blue sheep, it'll do the wololo sound and convert it to a red sheep.