Merrow Harbinger 

Creature — Merfolk Wizard
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
When this creature enters, you may search your library for a Merfolk card, reveal it, then shuffle and put that card on top.
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Cards that do something similar expected a card that isn't here?
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"When this creature enters, you may search your library for a Merfolk card, reveal it, then shuffle and put that card on top."