Heretic

Were you looking for the Halo 2 first level?

"Heretic returns players to familiar ground as the Halo 2 arena “Midship” makes its long-rumored debut in Halo 3. The stark interior of the Pious Inquisitor might appear austere on first glance, but Heretic’s symmetrical spaces require players to execute complex strategies in order to outplay their opponents."

- Bungie

Heretic is a Halo 3 DLC map that can be obtained by buying the Halo 3: ODST game disc, available on September 22nd, 2009. Heretic is officially a remake of the popular Halo 2 map, Midship, this was confirmed by Urk in the 6/05/09 weekly update on Bungie.net.

Skull
The crest of the hidden skull on this level appears to be the Mark of Shame.

Achievements
On Heretic, there are two achievements 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 is 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 rumors 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 confirmation that it was a real picture on Bungie.net.