Were you looking for the Halo 2 level or the Covenant belief?

The Great Journey is an achievement in Halo 2 Vista and Halo: The Master Chief Collection. It can be obtained by beating the level The Great Journey, on Normal, Heroic, or Legendary.

The Halo 2 Vista achievement is worth 30 Gamerpoints and is represented by the head of the Jiralhanae, Tartarus.