Helm of Obedience

[Variable Colorless], [Tap]: Target opponent puts cards from the top of his/her library into his/her graveyard until a creature card/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.