Gurmag Drowner

Oracle Text

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.

Card Rulings

2/25/2015 A creature with exploit “exploits a creature” when the controller of the exploit ability sacrifices a creature as that ability resolves.
2/25/2015 You choose whether to sacrifice a creature and which creature to sacrifice as the exploit ability resolves.
2/25/2015 You can sacrifice the creature with exploit if it’s still on the battlefield. This will cause its other ability to trigger.
2/25/2015 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.
2/25/2015 You can’t sacrifice more than one creature to any one exploit ability.