Decayed is a hyper-specific mechanic from Innistrad: Midnight Hunt, and appears solely on Zombie creature tokens. If a creature has the mechanic, that represents two different things: The creature cannot block, and if it ever attacks, it gets sacrificed at the end of that combat.
And that’s it. Once a Decayed creature attacks, it will get sacrificed at the end of combat even if it loses the ability somewhere in between. You don’t have to attack with a decayed creature, but once you do, it’s going to go away one way or another.
Cards other than the zombie tokens can get decayed, thanks to cards like Gisa, Glorious Resurrector and Ghouls’ Night Out, but other than those tokens no card comes with decayed naturally. Designed to allow for zombie creation at lower costs and rarities, it was received well enough if not with a ton of enthusiasm. That said, it did make for some interesting play patterns and decisions during games, and could see the light of day again someday in a plane where having lots of zombies makes sense.
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