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When Abyssal Gorestalker enters the battlefield, each player sacrifices two creatures.
As the enters-the-battlefield ability resolves, first the player whose turn it is chooses which two creatures they are going to sacrifice, then each other player in turn order does the same. Players will know the choices of previous players when making their choices. Then all creatures are sacrificed by their controllers simultaneously.
If a player controls only one creature as the ability resolves, they must sacrifice it.
You may sacrifice Abyssal Gorestalker to its own ability. If it is the only creature you control or one of two creatures you control, you must sacrifice it.
