Guardian Forest is a level that was cut from the final version of Halo 3.[1] The level would have introduced a new enemy called "The Guardian". At the end of the level, the player would have had to battle a giant Sentinel similar to the Enforcers from Halo 2. Part of the level later became the multiplayer map Guardian, while another part (that involved a Hornet air battle) was transferred to a completely different level, The Covenant.
Sources