Theft decks use opponents' cards and permanents against them. This can mean taking control of something already on the battlefield, stealing cards from libraries or graveyards, or temporarily borrowing creatures for one decisive turn.
Etali, Primal Storm casts cards from opposing libraries, while Mind Flayer takes control of a creature for as long as it remains around. Sacrifice outlets can make temporary theft effects such as Act of Treason considerably more permanent.
