Citadel

Citadel is an upcoming Halo 3 map that will be shipped in Fall 2009 with Halo 3: ODST. The Shotgun Spree achievement is attainable on this map. It is possible that this map is located in a Forerunner Citadel, similar to the one seen on The Covenant.

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 may take place on Forerunner terrain or, given the Covenant's use of Forerunner glyphs, is Covenant in aesthetics.