Marygold, the Innocent Victim

Marygold, the Innocent Victim can't attack or block.
Whenever a source an opponent controls deals damage to you, discard a card, then put a corruption counter on Marygold, the Innocent Victim. If if there are four or more corruption counters on Marygold, the Innocent Victim, exile it, then return her to the battlefield transformed under his owner's control.