From Halopedia, the Halo wiki
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In the original game, it is worth 20 Gamerscore and is unlocked by beating Sierra 117 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 I 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 Easy. In The Master Chief Collection it is represented by a screenshot of the level.