Guardian Forest

Guardian Forest was a planned level in Halo 3's campaign, intended to take place later in the campaign.

Description
Guardian Forest was a level intended to be set on Installation 00, in the eponymous forest. During the level, the Master Chief would have piloted an AV-14 Hornet through the trees in an aerial battle. Although Bungie scrapped Guardian Forest, the team felt the aerial battle was too fun to lose, and transplanted the sequence onto the next level, The Covenant. The level was also intended to introduce a new Forerunner enemy type, the titular Guardian. Described by producer Allen Murray as a "floating, conical sentry" with a laser-emitting eyeball that the player could rip out and use as a weapon.

Much concept art was created for the Guardian enemy, with the Guardian reaching a stage of being fully-modelled and animated, with the weapon and character itself extensively prototyped in-engine. The Guardian was intended to be massive in size, similar to the cut Strato-Sentinels. The Guardian model remained in the final release of Halo 3, existing as a hologram on the map Epitaph, while the Guardians' concept was later used and expanded upon for the Guardian Custode which debuted in Halo 5: Guardians. The Guardian concept from Halo 3 additionally featured in the Halo Legends episode Origins.

An early design document created by designer Dan Miller was posted on Twitter in 2017 (shown below).

This design document reveals a number of details Bungie had considered for inclusion in the level. Notably, the level was to include Strato-Sentinels and Enforcers as enemies alongside the Guardian, with flyable Hornets and Banshees. The level's story would involve Master Chief travelling through the forest to activate the Ark's defenses, as Bungie felt that simply having two levels to show the end of the Covenant wasn't enough. Instead, the level was intended to be similar to the New Alexandria level later featured in Halo: Reach, with the player going to three areas and activating a device in each one; when each device was primed, the Ark would release Constructor Sentinels, who would proceed to act as the Ark's defense system and destroy Truth's fleet in the space battle above.

During the level, the player would ride on a "train" for a section before the power died and the train ground to a halt. The player would then "crawl through the creepy forest" before eventually flying the aforementioned aircraft (presumably in the "New Alexandria"-esque segment mentioned prior). At some point in the level, the Guardian would be introduced with a dedicated vignette, and eventually the player would be able to rip out its eye and use it as a weapon. The level would close with the Strato-Sentinel rising out of the ground, with the player riding it.

By the time the level was cut, the Guardian Forest level had more than a year of art, design and engineering time put into it. Unfortunately, the level was one of many victims of scope reduction in Halo 3 for time. However, by this time many pieces of concept art had been put together for Guardian Forest by artists Chris Barrett and Frank Capezzuto, which inspired level designer Steve Cotton to create a map with lots of platforms and walkways similar to Halo 2's Lockout. Cotton put together a "moodier" piece of concept design, which led to the Guardian Forest aesthetic and design being repurposed for the Halo 3 multiplayer map Guardian.

Current status
Although most level files for Guardian Forest do not exist within Halo 3 anymore, one texture file does remain in the  folder, indicating that Guardian Forest was intended to take place following the level The Ark and before the level The Covenant (The Ark and The Covenant are known internally, respectively, as   and  ). However, a GDC talk given by Allen Murray lists the level as intended to take place after The Covenant and before the level Cortana.. Further insight by Dan Miller supports that the level was to take place after The Ark.