Demon (Halo 3 achievement)
From Halopedia, the Halo wiki
This article is about the Halo 3 achievement. For the Covenant term, see Demon. For the Halo 2 for Windows Vista achievement, see Demon (Halo 2 achievement).
The artwork of the achievement is represented by a blue circle with a horned skull. It is worth 10 Gamerscore and is earned when the player earns 15,000 points while playing the meta-game on Crow's Nest.
Method 1 - Turn on the Fog, Catch, Tough Luck and Grunt Birthday Party skulls. Using the Battle Rifle and Shotgun, headshot as many Grunts as possible and Eliminate/Melee as many Brutes as you can. Also for better scoring, try to race through the level as well as destroying enemies as fast as possible.
There are some theories as to its naming: the name ties to the name given to Spartans, specifically John-117. Or, it could be that the Master Chief must turn the tables on the Covenant intruders, who, at the start of the level, have penetrated almost all sections of the base. This might also be a reference to Truth's taunting in the beginning cutscene about how even the Demon will die at the Covenant's hands.