Citadel

Citadel is an upcoming Halo 3 map that will either be included in the second part of the Mythic Map Pack, or it will be shipped in Fall 2009 with Halo 3: ODST.

Skull
This map has a skull for the multiplayer achievements. When found it unlocks the achievement, Citadel Skull. It has an image on the skull that resembles a Forerunner glyph, indicating that it takes place on a Forerunner terrain or, given the Covenant's use of Forerunner glyphs, that it is Covenant in aesthetics.

Trivia

 * This map is likely be based on the end of the level The Covenant as where Truth, attempted to activate the Halos because it's called "The Citadel", but may be a re-modeled "Citadel" instead.
 * This may also be another map similar to Construct, but due to the Shotgun Spree achievement, it may be more of a close-quarters map.
 * Another theory is that this map takes place inside a Covenant Citadel on the Ark. A Covenant Citadel is also the main base for the Covenant in Halo Wars.