Muriel, the Stormtide

Islandwalk
Sirens and Merfolk you control have; whenever
this creature deals combat damage to a player,
put the top card of that players library into the graveyard.
Waxing Waves - If four or more cards were put
into an opponent's graveyard from a library this
turn, you may cast up to one creature card from
your graveyard.