Helm of Obedience

{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.