Helm of Obedience

Text:

, : 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.

Rulings:

• 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.