"Cleanse High Charity. Save Cortana."
Cortana is the ninth campaign mission of Halo 3. It takes place in the Flood infested High Charity. Your mission is to find Cortana in order to activate Installation 04B.
Upon completing it on the Normal, Heroic, or Legendary difficulties, the player will earn the "The Key" achievement and 40 Gamerpoints.
Earning over fifteen thousand points in the campaign meta-game on this level will give the player the Orpheus achievement, and 10 Gamerpoints.