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 an almost direct 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.

Speculation on the MLG Forums
"Heretic: Midship remake. The ONLY - and literally ONLY - difference any of us could find (we tested all the jumps) was that jump pad on top, where you can jump up and get the sword, it was bouncy whenever you jumped or threw grenades on it in H2. It wasn't bouncy in Heretic. Everything else was faithful. Programmed into the map is BR/plasma nade starts. The sky box when you look out the windows is AMAZING - it is a view of quite a few Covenant carriers and ships coming into possibly our Solar system, and you are looking out from the belly of another ship. There is one that is really close to you and you can see amazing detail on the ship. There is a planet nearby too."

Trivia

 * Heretic is a remake of Midship, and it will be the fourth direct remake in Halo 3. The others being Last Resort, Blackout, and Cold Storage. Avalanche is not a direct remake.
 * 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.
 * If you look in the first picture on this page, down from the red Spartan and to the left just a little bit, you can see what appears to be a Marathon symbol. Though this is only a reflection from the structure above it, it looks similar the the Marathon symbol none the less.