Silumgar Butcher

Exploit (When this creature enters, you may sacrifice a creature.)
When this creature exploits a creature, target creature gets -3/-3 until end of turn.

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Dragons of Tarkir

Exploit (When this creature enters the battlefield, you may sacrifice a creature.)
When Silumgar Butcher exploits a creature, target creature gets -3/-3 until end of turn.

A creature with exploit “exploits a creature” when the controller of the exploit ability sacrifices a creature as that ability resolves.

If the creature with exploit isn’t on the battlefield as the exploit ability resolves, you won’t get any bonus from the creature with exploit, even if you sacrifice a creature. Because the creature with exploit isn’t on the battlefield, its other triggered ability won’t trigger.

You can sacrifice the creature with exploit if it’s still on the battlefield. This will cause its other ability to trigger.

You can’t sacrifice more than one creature to any one exploit ability.

You choose whether to sacrifice a creature and which creature to sacrifice as the exploit ability resolves.