Okeanos, Lord of the Trench

Hexproof, Islandwalk
Leviathan, Kraken, Serpent, and Octopus
spells you cast cost {4} less to cast
Whenever Okeanos attacks, you may sacrifice
another Leviathan, Kraken, Serpent, or Octopus.
If you do then Okeanos gets +X/+X where X is
the sacrificed creature's power.