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: 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.
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).