Larsk, Trench Seeker

This creature cannot be blocked.
On each player's upkeep, sacrifice a creature that is not a Fish, Serpent, Kraken, or Leviathan.
Fish, Serpents, Krakens, and Leviathans you control cost {1} less to cast for each card in your graveyard.
While Opalash ambushes from the surface, Larsk ambushes from below.