Cheapest priced copy (Alliances): $32.99 at Card Kingdom, near mint, their price list as of .
Helm of Obedience 
Artifact · Rare · Alliances · #121
{X}, {T}: Target opponent mills a card, then repeats this process until a creature card or X cards have been put into their graveyard this way, whichever comes first. If one or more creature cards were put into that graveyard this way, sacrifice this artifact and put one of them onto the battlefield under your control. X can't be 0.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $32.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Alliances | 121 | Rare | $32.99 | — |
2 rulings from Wizards of the Coast
- 1 October 2008If 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.
- 1 October 2008You 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).