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.

Speculation
This map could be a remake of a popular Halo 2 map, Midship. It takes place on a Covenant Ship, possibly Pious Inquisitor. Expert players were invited to play on the beta version of Heretic, Longshore, Orbital, Assembly and Citadel but not Sandbox. Most say it's slightly different from Midship, but this can't be confirmed as of yet. This map could also be a remake of Colossus or Burial Mounds.

Skull
The crest of the hidden skull on this level appears to be the Covenant Mark of Shame. Judging by this, as well as by the name, it seems that the level will be set in a Covenant Separatist environment.

Trivia

 * If it is true that Heretic is a remake of Midship, it will be the fifth remake in Halo 3. The others being Last Resort, Blackout, Avalanche, and Cold Storage.
 * 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 or The Heretic 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.
 * There is a hidden room (via theater mode) on the campaign level "The Ark" that May in fact be Heretic. This is mearly speculation, as the room was seen on the last cutsceen in the level "Floodgate", and later covered in Flood biomass on the level "Cortana".