Cleansing is an achievement for Halo 3 and Halo: The Master Chief Collection. In the original game, it is worth 30 gamerpoints and is unlocked by beating Floodgate on Normal, Heroic, or Legendary. In The Master Chief Collection it is worth 10 gamerpoints and can be unlocked on Easy.
In the original game it is represented by a blue plate, with a roman numeral V on it. In The Master Chief Collection it is represented by a screenshot of the level.
The level must be played from Mission Start in order to unlock the achievement. Playing in co-op will still earn the achievement.