Crow's Nest

Holdout is believed to be the second campaign mission of Halo 3.

It takes place in a UNSC Base.

From footage of the halo 3 campaign in the brute videdoc, it looks like you play as the arbiter in this mission.

Upon completing it on the Normal, Heroic, or Legendary difficulties, the player will earn the "Holdout" achievement and 20 Gamerpoints.