Helm of Obedience ★★★★★

{X}, : Target opponent puts cards from the top of his or her library into his or her graveyard until a creature card or X cards are put into that graveyard this way, whichever comes first. If a creature card is put into that graveyard this way, sacrifice Helm of Obedience and put that card onto the battlefield under your control. X can't be 0.