From Halopedia, the Halo wiki
Halo is an achievement in Halo: Combat Evolved Anniversary and Halo: The Master Chief Collection. It is awarded for completing the level Halo on any difficulty. The achievement is worth 25 Gamerscore in Combat Evolved Anniversary and 10 Gamerscore in The Master Chief Collection.
In the original Halo: CE Anniversary it is represented by the Roman numeral of "II" over a darkened version of Anniversary's logo. In The Master Chief Collection it is represented by a screenshot of the cliff edge section of the level.