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 officially a remake of the popular Halo 2 map, Midship. It takes place in a Covenant Ship, like its predecessor. On June 2nd, 2009, Bungie Employee Joe Staten announced that Heretic is a Midship remake.

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.

Achievements
On Heretic, there are two achievments available to get. The first being the hidden skull which will award you 25 Gamerscore and the second being Blades of Fury - Get a Sword Spree on Heretic (Ranked or Social) will also award you 25 Gamerscore.

Trivia

 * Heretic will be the fifth remake/reimagination in Halo 3; the others being Last Resort, a remake of Zanzibar, Blackout a remake of Lockout, Avalanche, a remake of Sidewinder and Cold Storage, a remake of Chill Out.
 * There were rumours that the image of Heretic was a hoax as it was released close to April Fool's day, however this has since been disproved with the conformation that it was a real picture on Bungie.net.