This article is too damned short, Marine! You will help Gruntipedia by so it will fit the screen, fool!
The Guardian Forest is a level that was cut from Halo 3. The player or Noob would have been introduced to a enemy (yet another thing being hostile for no reason) called "The Guardian". At the end, though, is where the player would have to battle a giant Sentinel. Since Bungie didn't believe in boss levels, they just cut this level in half. A part of it is now the level Guardian and the horny air battle that was going to be in the level is now in the Covenant campaign level.