From Halopedia, the Halo wiki
In the original game, it is worth 20 Gamerscore and is unlocked by beating Tsavo Highway 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 III 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.