Archangel Avacyn, Malevolent Whispers; what happens?
Since Anton controlled Avacyn when the creature died, they control the delayed trigger. Even though they don't control Avacyn, Avacyn will still transform into Avacyn, the Purifier (there's no rule that says you have to control the permanent to be able to transform it). Since the opponent controls Avacyn when it transforms, they control the Purifier's trigger.
Nathan Long
Magic Rules NetRep