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When Slum Reaper enters the battlefield, each player sacrifices a creature.
As Slum Reaper’s ability resolves, first the player whose turn it is chooses a creature they control, then each other player in turn order does the same, knowing the choices made before them. Then all the chosen creatures are sacrificed at the same time.
If any player controls no creatures, that player simply doesn’t sacrifice anything. Each other player still sacrifices a creature.
Slum Reaper may be the creature its controller sacrifices for its own ability.
