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.

Overview
This map is a remake of a popular Halo 2 map, Midship. It takes place in the Covenant Ship, Pious Inquisitor.

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.
 * 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.