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When Ravenous Rotbelly enters the battlefield, you may sacrifice up to three Zombies. When you sacrifice one or more Zombies this way, each opponent sacrifices that many creatures.
After you sacrifice one or more Zombies, a second ability triggers. Players may respond to it before they have to sacrifice creatures.
As the reflexive triggered ability resolves, first the next opponent in turn order (or, if it's an opponent's turn, the opponent whose turn it is) chooses the creatures they plan to sacrifice. Then each other opponent in turn order does the same. Finally all chosen creatures are sacrificed at the same time.
