Helm of Obedience

Helm of Obedience {4}

Artifact (CMC 4)
{X}, {T}: Target opponent puts cards from the top of their library into their 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.