Hidden Horror

Oracle Text

When Hidden Horror enters the battlefield, sacrifice it unless you discard a creature card.

Card Rulings

4/1/2008 When the ability resolves, you choose whether to sacrifice the creature or perform the other action. If you can’t perform the other action, then you must sacrifice the creature.
4/1/2008 If the creature is no longer on the battlefield when the ability resolves, you may still perform the action if you want.