The Key

The Key is a Campaign Achievement for Halo 3 and Halo: The Master Chief Collection.

Halo 3
In the original game, it is worth 40 Gamerscore and is unlocked by beating Cortana from the start of the mission on Normal, Heroic, or Legendary. In the original game it is represented by a blue plate with the Roman numeral VIII on it.

Halo: The Master Chief Collection
In The Master Chief Collection it is only worth 10 Gamerscore, though it can also be unlocked on any difficulty. The achievement's Xbox art is the loadscreen image for the level.