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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.
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.
