Decayed represents a static ability and a triggered ability. "Decayed" means "This creature can't block" and "When this creature attacks, sacrifice it at end of combat."
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Decayed does not create any attacking requirements. You may choose not to attack with a creature that has decayed.
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Decayed does not grant haste. Creatures with decayed that enter the battlefield during your turn may not attack until your next turn.
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Once a creature with decayed attacks, it will be sacrificed at end of combat, even if it no longer has decayed at that time.
Flying
When this creature enters, create a 2/2 black Zombie creature token with decayed. (It can't block. When it attacks, sacrifice it at end of combat.)
