Mesmeric Fiend

When this creature enters, target opponent reveals their hand and you choose a nonland card from it. Exile that card.
When this creature leaves the battlefield, return the exiled card to its owner's hand.

The List

Masters 25

When Mesmeric Fiend enters the battlefield, target opponent reveals his or her hand and you choose a nonland card from it. Exile that card.
When Mesmeric Fiend leaves the battlefield, return the exiled card to its owner's hand.

Vintage Masters

Duel Decks: Sorin vs. Tibalt

Torment

When Mesmeric Fiend comes into play, target opponent reveals his or her hand and you choose a nonland card from it. Remove that card from the game.
When Mesmeric Fiend leaves play, return the removed card to its owner's hand.

If Mesmeric Fiend leaves the battlefield before its first ability has resolved, its second ability will trigger and do nothing. Then its first ability will resolve and exile a nonland card from the target opponent's hand indefinitely.