The Covenant (level)

Last Stand is believed to be the seventh campaign mission of Halo 3.

It takes place in a citadel.

Upon completing it on the Normal, Heroic, or Legendary difficulties, the player will earn the "Last Stand" achievement and 40 Gamerpoints.

Trivia
in which you must Venture into the Citadel and learn the secrets of the S'pht.
 * Marathon: Durandal has an achievement called '11th Clan of the S'pht'