When a creature with undying dies (without any +1/+1 counters), it returns to the battlefield under its owner’s control, even if it changed controllers before it died. The “owner” of a card is the player who started the game with the card in his or her deck (the legal owner of the physical card is not relevant to the game itself). So if you use Traitorous Blood on your opponent’s Flayer of the Hatebound and it dies, when the undying trigger resolves, it will be returned under your opponent’s control, even though you controlled it when undying triggered. This is because undying only cares about the owner, not the controller of the creature.

Today’s Rules Tip written by
Peter Golightly, Level 1 Judge from Austin, TX