Cleansing is a Campaign Achievement for Halo 3 and Halo: The Master Chief Collection.
It is obtained when someone finishes the Campaign mission Floodgate on the Normal, Heroic, or Legendary difficulty.
The Halo 3 achievement gives 30 Gamerpoints to the player and is represented by a blue plate, with a roman numeral V (five) on it.
The level must be played from Mission Start in order to unlock the achievement. Playing in co-op will still earn the achievement.