Sheoldred's Terror is a uncommon Instant card from Unknown Event (UNK), collector number UB12.
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!)