Ashiok's Nightmare Beast

Skulk
Whenever Ashiok's Nightmare Beast deals combat damage to a player, you may pay {1}{u}{b} any
amount of times. For each time that cost is paid,
that player puts the top ten cards of his or her graveyard into his or her library.
{2}{u}{b}: Ashiok's Nightmare Beast is unblockable
until end of turn.