From Halopedia, the Halo wiki
In the original game, it is worth 50 Gamerscore and is unlocked by beating Halo 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 IX 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.