Heretic

Were you looking for the Halo 2 first level or the Heretic Uprising?

Heretic is a Halo 3 DLC map that can be obtained by buying the Halo 3: ODST game disc, available in Fall 2009.

Design
Rumored to be a remake of the very popular Halo 2 map, Midship. All jumps and been tested and performed, the only difference that the testers at Bungie have confirmed is the top platform which hold the sword does not move like it used to. Rumor states this map starts with Battle Rifles and Plasma Grenades by default. The view of this map is from the belly of another ship, looking through the windows you can see Large covenant carriers far out in the distance. There is also a nearby planet.

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 Campaign 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, as Colossus was a remake of The Arbiter.
 * 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.