Mark of the Oni

Theft auras live in blue, where Control Magic and Mind Control hand you the body permanently and charge for it in mana and color. This one ports the effect into black and then attaches a string: the control change holds only while you keep a Demon in play, and the end-step check enforces it with no grace period. Lose your last Demon and the Aura sacrifices itself at the very next end step, handing the creature back to its owner. That clause is the whole engine. It prices a generic steal against a tribal commitment, turning a control effect into a payoff that fires only inside a Demon shell, and it builds in a clean failure state: an opponent can break the theft by killing your Demon rather than the Aura, since either one ends the arrangement. It belongs to an era when efficient effects were routinely gated behind a creature type you already had to be playing, rewarding the dedicated deck and offering the splash nothing. The end-step condition also makes the body genuinely yours while you hold it: any control-change effect lets the new controller sacrifice the creature to their own outlets, so feeding the stolen creature to a sacrifice engine leaves the Aura with nothing to enchant and no debt owed. The trade against blue's permanent theft is plain enough: cheaper and in-color, but only as durable as your Demon count.