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When Fleshbag Marauder enters the battlefield, each player sacrifices a creature.
As Fleshbag Marauder's ability resolves, first the player whose turn it is chooses a creature to sacrifice, then each other player in turn order does the same knowing the choices made before them. Then all those creatures are sacrificed simultaneously.
When its ability resolves, you may sacrifice Fleshbag Marauder itself. If you control no other creatures, you'll have to sacrifice Fleshbag Marauder.
