Instant
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You don't have to choose any targets for Rotten Reunion. However, if you do, and that target is illegal as Rotten Reunion tries to resolve (perhaps because your opponent responded with their own Rotten Reunion), it will be removed from the stack and you won't create a Zombie token.
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."
Decayed does not create any attacking requirements. You may choose not to attack with a creature that has decayed.
Decayed does not grant haste. Creatures with decayed that enter the battlefield during your turn may not attack until your next turn.
Once a creature with decayed attacks, it will be sacrificed at end of combat, even if it no longer has decayed at that time.
(You may cast this card from your graveyard for its flashback cost. Then exile it.)