Bonegorger Leviathan

Bonegorger Leviathan doesn't untap during your untap step.
{4}{u}{u}: Untap Bonegorger Leviathan. Activate this ability only as a sorcery.
Whenever Bonegorger Leviathan becomes untapped, have target player put the top 5 cards of his/her library into his/her graveyard. Then return Bonegorger Leviathan to its owner's hand at the beginning of your endstep.