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
Heretic is possibly a remake of the popular Halo 2 map, Midship. It takes place in a Covenant Ship, like its predecessor.

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 Heretic is a remake of Midship. It will be the fifth remake in Halo 3. The others being Last Resort, Avalanche, Blackout, and Cold Storage.
 * 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.
 * The leaked picture of Heretic may be a hoax because it was leaked close to April Fools day.