Omniscient Demon Slayer

Omniscient Demon Slayer may only attack a
player as long as he or she controls a demon. At
the beginning of your upkeep you may have
target opponent gain control of a demon that is
under your control. Demon creatures under your opponents control that you own gain, At the beginning of your end step, this creature deals x
damage to you, where x is equal to its mana cost.