Helm of Obedience

Artifact, 4 (4)
{X}, {T}: 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 0.¹
Illus. Brian Snõddy

Gatherer Card Rulings [Nov 2009]

• 10/1/2008: You put the creature card onto the battlefield even if you can't sacrifice Helm of Obedience (because it's left the battlefield by the time its ability resolves, for example).
• 10/1/2008: If an effect like that of Leyline of the Void prevents cards from being put into your opponent's graveyard, the process described in the first sentence of Helm of Obedience's effect will never stop. Your opponent's entire library will be exiled, even if X is 1.

Format Legality	Legal in Vintage, Legacy, MTGO Classic Banned in Extended, Standard
Printings (2/7)	Alliances (Rare) MTGO Masters Edition II (Rare)