As an additional cost to cast Collateral Damage, sacrifice a creature.
Collateral Damage deals 3 damage to target creature or player.
11/24/2014 If you sacrifice an attacking or blocking creature during the declare blockers step, it won’t deal combat damage. If you wait until the combat damage step, but that creature is dealt lethal damage, it’ll be destroyed before you get a chance to sacrifice it.
11/24/2014 You must sacrifice exactly one creature to cast this spell; you can’t cast it without sacrificing a creature, and you can’t sacrifice additional creatures.
11/24/2014 Players can respond to Collateral Damage only after it’s been cast and all its costs have been paid. No one can try to destroy the creature you sacrificed to stop you from casting this spell.