Doppel

When Doppel comes onto the field, she is able to
steal an opponent's planeswalker or creature, or you
will be able to find any card you desire in your
deck, but if you search for that card, you will need
the deck reshuffled.

Once the thing that she stole is dead, she can immediately do it again, so she can steal another planeswalker or creature if the original dies, this continues until the game if over or she gets sacrificed.