It's for Diremaul north, when you kill their king, you will become their new king (and they're friendly). This is the buff to turn their names green. ;)
I stealthed this at 70 with another rogue and a mage, and when we killed the king, we couldn't get the buff from mizzle... Is there a level limit to it or something?
As of 2.4, it counts as a debuff, lasting until you leave the instance. If you wander back through and find any ogres still alive, you can talk to them.
One of them paraphrases "When you leave, another ogre will take your place. That's the way of the Gordok"... explains the scenario well.