Shadow Leviathan

Indestructible, hexproof, protection from black
This creature cannot be played unless a card named Devil's Son is in a graveyard. When this creature enters the battlefield, gain control of target creature. Its power and toughness is added to Shadow Leviathan. Shadow Leviathan's power and toughness is added to targeted creature. If Shadow Leviathan were to be sent to the graveyard, the targeted creature is sent instead.
"I can feel it coiling around my being and soul, crushing for control." -Babel