Enchantment, 3U (4) All creatures have "At the beginning of your upkeep, sacrifice this creature unless you pay {1}."1 As insubstantial-and as insistent-as appetite
Illus. Andrew Robinson
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.