Heretic

Heretic may be an upcoming map in the Halo 3 Mythic Map Pack.

Skull
The crest of the hidden skull on this level appears to be the Covenant Mark of Shame. Judging by this, it seems the level will have a Covenant environment or the Flood controlled High Charity.

Trivia

 * The term "Heretic" is used by the Covenant Loyalists to refer to the Covenant Separatists, hinting that this map may feature Covenant Separatist structures. This map could also be based on the level The Arbiter from Halo 2, as the Gas Mining Facility featured in that level was controlled by the Heretics, and thus could be a remake of the Halo 2 map Colossus.
 * Getting a Sword Spree medal on this map earns the player the 'Blades of Fury' achievement. This fact suggests that it is either a small or medium sized map.
 * This map, Longshore and Citadel are not confirmed to come out with the Mythic Map Pack. Go here for details.