Gurmag Drowner

Exploit (When this creature enters, you may sacrifice a creature.)
When this creature exploits a creature, look at the top four cards of your library. Put one of them into your hand and the rest into your graveyard.

The List

Dragons of Tarkir

Exploit (When this creature enters the battlefield, you may sacrifice a creature.)
When Gurmag Drowner exploits a creature, look at the top four cards of your library. Put one of them into your hand and the rest into your graveyard.

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.