Ichor Explosion

Oracle Text

As an additional cost to cast this spell, sacrifice a creature.

All creatures get -X/-X until end of turn, where X is the sacrificed creature's power.

Card Rulings

6/1/2011 X is the power of the sacrificed creature as it last existed on the battlefield.
4/15/2013 You must sacrifice exactly one creature to cast this spell; you cannot cast it without sacrificing a creature, and you cannot sacrifice additional creatures.
4/15/2013 Players can only respond once this spell has been cast and all its costs have been paid. No one can try to destroy the creature you sacrificed to prevent you from casting this spell.