Sheoldred's Terror (UNK-UB12) - Unknown Event

Destroy target creature not originally printed in an Argentum expansion. (Mirrodin, Darksteel, Fifth Dawn, Scars of Mirrodin, Mirrodin Besieged, New Phyrexia, and Phyrexia: All Will Be One are Argentum expansions. Argentum is Karn's original name for that plane, now you know!)