When Mesmeric Fiend enters, target opponent reveals their 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.
2018-03-16
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.