Gurmag Drowner

R2.10

“A creature with exploit exploits a creature when the controller of the exploit ability sacrifices a creature as that ability resolves.
You choose whether to sacrifice a creature and which creature to sacrifice as the exploit ability resolves.
You can sacrifice the creature with exploit if it’s still on the battlefield. This will cause its other ability to trigger.
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’t sacrifice more than one creature to any one exploit ability.”