Seithenyn, The Drowned King

Drowned creatures you control get +1/+1.
Whenever Seithenyn, The Drowned King deals
combat damage to a player, you may cast target instant or sorcery card from that player's
graveyard without paying its mana cost. If that
card would be put into a graveyard this turn,
exile it instead.
If a card would be put into your graveyard, it is
put on the bottom of your library instead.
The Ocean hid something. Something that should not
have been forgotten.