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 is almost confirmed to 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 its a slightly different midship but can't be confirmed as of yet.

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

 * 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.