Guardian Forest

080_forest, or more colloquially Guardian Forest, was a level planned for inclusion in the campaign Halo 3, though was ultimately one of many cut from the game. In the original intended level sequencing, 080_forest was to have succeeded 070_waste (The Ark) and preceded 090_alpine (Alpine - a cut level level that became The Covenant in the final game). A GDC talk given by Halo 3 producer Allen Murray lists the level as intended to take place after The Covenant and before the level Cortana - however, this is likely in error and not supported by any other source..

In the originally-planned story for Halo 3, outlined in a first-draft script uploaded by campaign lead Rob Stokes and a series of storyboards uploaded by Lee Wilson, Guardian Forest would have served to introduce the Guardian Sentinels and Strato-Sentinels.

Overview
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 Sentinel. 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.

A photograph of a digital design document was posted on Twitter by Dan Miller 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.

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 though was noted by Dan Miller as still being cut incredibly early in Halo 3's development. 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.

Developers known to have worked on Guardian Forest include:
 * Rob Stokes (mission design and script)
 * Dan Miller (level designer)
 * Chris Barrett (concept art)
 * Frank Capezzuto (concept art)
 * Isaac Hannaford (concept art paintovers)
 * Lee Wilson (storyboards)

Background
Guardian Forest was to follow on from the level The Ark. As such, the original outlines for the Ark depict a slightly different ending to the one featured in the final game. As in the final level, the player would have attacked the Cartographer to access the schematics for the Ark. However, by this point in time, the ongoing space battle in the skies above would be going badly for the alliance, with Truth's fleet owning the skies and forcing the surviving allied vessels to go to ground. With the Prophet of Truth secured in the Ark's core behind a series of barriers, John-117, the Arbiter, and 343 Guilty Spark would instead investigate using the Ark's native transportation systems to traverse the installation to deactivate the barrier towers protecting Truth.

While discussing the plan, Spark would mention that the route would necessitate passing through a region of the Ark termed the "Veneration Zone", and that he was not sure how the "Guardians" (referring to the level's eponymous enemy, the Guardian Sentinels) would respond. Unfortunately, the monitor would be cut off by a Phantom attack, and the final combat encounters of the level play out as usual. The next major departure comes at the end of the level; rather than boarding Johnson's D77H-TCI Pelican dropship, the player would instead board a "Forerunner train" (as-termed in the Guardian Forest design layouts) and begin the next level.

According to artist Paul Russel, the "train station" was ultimately retooled into the final game's mini-boss fight with the Brute Chieftain, Cethegus.

080_LA_TRAIN_ARRIVES
The specifics of the Guardian Forest mission are somewhat harder to pin down than other missions, as the released design layouts don't specifically label parts of their maps to correspond with the listed gameplay themes.

The mission would begin with the Master Chief on the train boarded in the prior level, with the design document listing that the train would come screeching to a halt - something elaborated by the script as an "unplanned stop in the forest". During the "train" section, multiple other trains bearing Covenant troops would pull up and alongside the player, described by Miller as a faster version of the gondola rides from Halo 2 level Regret, with the player having to adapt to ever-changing geometry. This section was later described by Dan Miller as having having geometry that would "come together in parallel and disconnect", "creating and removing gameplay space in real time". Some time after this, the player would "crawl through the creepy forest" before ascending a massive tree-structure to find the titular Guardian enemy as a mini-boss of sorts. 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.

While searching for a way out of the forest, the player would be informed that the stop could be fortuitous - the Veneration Zone controls the Sentinel defenses for the Ark overall, and an opportunity is present to activate them and turn the Ark's weapons on the Covenant fleet in orbit. As such, the objective changes slightly and the Chief begins climbing through the forest to reach the control point.

Towards the level's end, the player would be tasked with activating three generators to activate the Ark's defences. This section was intended to be similar to the New Alexandria level later featured in Halo: Reach, with the player flying an aircraft to get to the three areas and activate a device in each one; when each device was primed, the Ark would activate its defences and release Sentinels, who would proceed to act as the Ark's defence system and destroy Truth's fleet in the space battle above. As per the design documentation, this would have served as the primary air combat encounter for the game, with the AV-14 Hornet and Banshee both intended to be flyable for the section. Ultimately, the Hornet gameplay was instead moved onto The Covenant when Guardian Forest was cut as Bungie felt the gameplay was too fun to lose, and the levels Alpine and Forerunner City merged to form The Covenant.

Once the last of the three generators were activated, the chamber would seal shut and trap the player inside. At this point, it would be revealed that the player was not inside a Forerunner building but instead a vast Strato-Sentinel - which would proceed to begin rising from the ground. Other similar Sentinels would begin rising out of the ground too and begin taking on the Covenant starships above, destroying them and dragging them down to the ground. Seeing their own Sentinel heading for a Covenant ship, the player would then be prompted to run and jump out of the Sentinel into a dropship piloted by Sgt. Johnson.

Some alternate takes on this ending have also suggested the Sentinel fleet may have included Constructors and/or Enforcers (though these may also be errors in recollection on the developers' parts). The Strato-Sentinel rising out of the ground was given some concept treatment by artist Isaac Hannaford.

080_LB_ACTIVATED_DEFENSE
The ensuing cutscene would play once the player had jumped out of the Sentinel, in a fashion akin to the Lich boarding in the Halo 4 level Shutdown or the Pelican boarding in the levels Evacuation and Spire from Halo 5: Guardians and Halo Infinite, respectively. Master Chief would fall out of the sky and land onto Johnson's Pelican, swinging himself around and climbing into the troop bay to join Johnson, Arbiter, and a few Marines. The Pelican would fly away from the battle with the trio admiring the carnage, before nearly being hit by a Sentinel flying up out of the clouds. Johnson would then affirm that the team have a shot at the barriers surrounding Truth, and would request to rendezvous with (a stand-in for the final game's ) and.

The level would then end, and the next level Alpine would begin.

Current status and legacy
Although most level files for Guardian Forest do not exist within Halo 3 anymore, one texture file does remain in the  folder. A single script file remains in the  folder of the Halo 3 Editing Kit; while the file does not reveal much due to the level being cut early in development, it does refer to encounters for areas called "Sentinel Rooms".

The model for the Guardian enemy remains in Halo 3 as a hologram on the map Epitaph, though this was noted by Miller to have no particular significance beyond the multiplayer team liking the look of the design. This model was later canonised in the Halo Encyclopedia (2022 edition) as the definitive look for the ancilla Offensive Bias, and featured in the Halo Infinite campaign as a hologram. The Guardians' general 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.