Leviathan, Tyrant of the Abyss

Islandwalk
Morbid- Whenever another creature is dealt damage
by Leviathan and dies, put a flood counter on
Leviathan.
Whenever Leviathan deals damage to a player,
move all flood counters from Leviathan onto defending player, if a player has 7 or more flood counters at the beginning of the endstep, they
lose the game.