When Mesmeric Fiend enters the battlefield, 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.
3/16/2018 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.