Citadel

Citadel is an upcoming Halo 3 map that will be included in the Mythic Map Pack. Little is known about the map but it's name may suggest ties to a building similar to the Citadel in the Ark.

Skull
The crest of the hidden skull on this level appears to have some sort of a Forerunner symbol on it.

Trivia

 * This map is expected to be released with Assembly, Sandbox, Space Camp, Longshore, and Heretic in the Mythic Map Pack.
 * This map contains a hidden skull. Locating the skull earns the player the 'Citadel Skull' achievement.
 * Getting a Sword Spree medal on this map earns the player the 'Blades of Fury' achievement.