Helm of Obedience

A mill engine that secretly steals creatures, designed around a self-terminating loop that stops the instant it finds something worth taking. The clever part is the structural trade: you set X to bound how deep you dig, the card walks the opponent's library one card at a time, and the moment a creature card surfaces, the process halts, the Helm is sacrificed, and that creature arrives under your control. You are paying mana and an artifact to convert the first creature you turn up into your own, with the depth of the search priced by X (and the result as random as whatever sits near the top of their deck). The design lives entirely in the stopping condition: the loop ends when a creature is "put into the graveyard this way," or when X cards have gone there, whichever comes first.

That termination clause is also the seam a famous interaction pries open. Pair the Helm with Leyline of the Void, which replaces every mill with exile rather than a trip to the graveyard. Because no card is ever "put into the graveyard this way," the loop never meets its creature condition and never tallies cards toward X either, so it grinds through the entire library at any X greater than zero, decking the opponent outright. The base card is a slow, fair piece of larceny; reading its terminating condition closely is where the whole thing turns lethal.