The Final Grunt is found cowering on the edge of a cliff right before the Forward Unto Dawn section on the final level in Halo 3. If the player approaches him, he will repond with "Hey demon! The jerk store called and they're all outta you. Poor you, stolen at the age of six and conscripted into the military, unnhh! Okay look, you let me live, I got the Fist of Rukt, I'll be the bottom. I'll polish your boots, I'll polish your helmet. Its the gas, haha! When I'm off the gas, I don't know what I'm doing half the time."
However, discovering the "Final Grunt" leaves the player unable to complete the end of the level (and the game), because there will be no route to the end since the platforms will have fallen.