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The Forerunner is the translated name for the ancient race of enlightened beings that were the creators and builders of the Halos, the Ark, the Shield Worlds, Onyx, along with the artifacts that were found on the planets Reach, Earth, Sigma Octanus IV, Harvest, and Arcadia. All that is known about the Forerunner has been gleaned from these various artifacts. The predecessors of the Forerunner are thought to be another alien race, known as the Precursors. The alien faction known as the Covenant worshiped the Forerunners as gods, deriving much of their technology from Forerunner artifacts found throughout the galaxy, and believing any who tampered or destroyed these relics to be heretics. The Forerunners themselves were a bipedal species with five-fingered hands, besides this not much is known about their anatomy.
The Forerunners were a very advanced civilization, coming to power after the disappearance of another race, known to them as the Precursors. The Forerunners reached their peak before activating the Halo Array around 100,000 years ago, exactly as planned.
Being the only known race ever to achieve Tier 1 technology based on the Technological Achievement Tiers measurement, they were only superseded by the mysterious Precursors at Tier 0; evidence of which has been found on various planets. The fact that the Halos were scattered around the galaxy also hints at the possibility that the Forerunner Empire covered most, if not all, of the galaxy. They even went as far as creating a Mantle to protect all life in the galaxy, believing it was their role to protect due to them being so advanced. Later in their history, a Forerunner survey team came into contact with an extra-galactic parasite known as the Flood on the planet G 617 g (details of this first contact are unavailable). After this first contact, the Forerunner realized the great danger that the Flood posed to the entire galaxy and attempted to contain them. The attempt to contain the Flood was a failure, primarily because in the beginning, they treated the Flood as a disease, attempting to contain it and quarantine it, instead of treating it as a severe threat and declaring an all out war on the Flood. They did not start war until it was far too late. 
When the Forerunners were unable to contain the Flood outbreak, war between the two factions broke loose. During the 300 years of conflict, the Forerunner studied the Flood in labs such as the gas mine located in the atmosphere of Threshold, looking for some sort of weakness. Eventually they created the Halo Array, a weapon of last resort designed to starve the Flood to death by killing all sentient life with enough biomass to sustain them. 
At first, the Forerunners were reluctant to use the Array, believing that they should continue to embrace the Mantle and to protect life not destroy it. The Gravemind managed to convince a Contender-class AI, Mendicant Bias, to unite with the Flood. Mendicant Bias soon led his massive fleet in an attack on the "Maginot Line", the final barrier between what the Forerunners could protect and what they had to let go. The Forerunners were given no choice but to activate the Halo rings, indexing most intelligent lifeforms such as Humans, Sangheili and so on, killing themselves and all life of sufficient biomass and cognitive ability in the Milky Way, with the exception of those species safely placed on the Ark.
Forerunner technology was highly cultivated and progressive. They managed to create solidified surfaces out of meticulous energy, construct a Halo-wide teleportation grid and create and utilize a number of other advanced machinery. The Halo Rings themselves, the Micro-Dyson Mark II sphere, which is connected through a portal to the shield world Onyx, and the Ark (Installation 00) are, above all others, the most significant pieces of Forerunner technology. The planet Onyx in particular demonstrates both their ability for engineering on a grand scale, and their near transcendent grasp of Slipspace technology. It is unknown how much of their technology is based on the technology of the Precursors if any.
Covenant Luminaries (which are actually stolen Forerunner equipment designed to detect other Forerunner objects) can detect Humans, previously mislabeled as "Forerunner Artifacts", leading to the initial Human-Covenant contact which sparked a seemingly endless war. Ironically, the Covenant worships the Forerunners but fight Humans, who have been confirmed as Forerunner descendants by the Oracle. The idea of living Forerunners in the Galaxy (described by the Oracle as Reclaimers) terrified the Covenant leadership. In an attempt to protect their religion the Prophets of Truth, Regret and Mercy concealed the true identity of Humanity to the rest of the Covenant and deemed them as heretics, thus sparking the Human-Covenant War.
Huragok are the only known Forerunner creation to be both non-robotic and to join the Covenant.
Forerunners have the technology to reproduce entire individuals from encoded DNA/RNA/silicon samples in data streams. This was mentioned in the Bestiarum: "DNA/RNA/Silicon samples are encoded in this data stream. Reproduction of individuals for analysis is prohibited by this facility."
The Forerunners appeared to be genetically related to the humans, which can be corroborated by the Iris Server-05 message about the humans providing "answers" to the Forerunner's own "mysteries", and Mendicant Bias's own declaration in Halo: Contact Harvest. This also explains why the Prophets ordered the Covenant to exterminate humanity, knowing that they are the Forerunner's designated heirs and would undermine their power. Halo 3 and the Halo Encyclopedia provided further evidence of this as well.
Forerunner constructs and sentries use a broad variety of weapons, typically high-energy powered beams. These weapons proved to be very effective when battling the Flood, burning Flood forms to the point that they cannot be revived by their compatriots. The Forerunners installed and constructed these intricate beam weapons inside their paradigm Arrays, allowing them to build an army of machines to battle the Flood without sending any Forerunner to the front lines. Forerunners evidently made active use of plasma weaponry as well; the Covenant's own weapons and technology are solely based on Forerunner artifacts.
The Sentinels wield orange-colored directed energy beams, used primarily for fighting the Flood. Sentinel Majors however, have a more powerful and accurate version of the beam, which sports superior energy output, at the cost of overheating issues. These more advanced energy beams are colored blue. All variants of these Sentinel Beams can be wielded by Humans and certain Covenant races.
Meanwhile, Enforcers are equipped with multiple packs of small grenades, or mortar-like explosives launched over the top of their shield. The Enforcer also boasts a pair of Pulse Beams, which fire clusters of smaller red Needler-like projectiles or bolts of energy at an incredibly high rate of fire. These pulse beams are primarily used against infantry at close range. These mammoth constructs also seem capable of lifting even tanks, using some sort of magnetic or anti-gravity grapple, before crushing the target between their massive 'arms'. The Constructors' repair beam can also be considered a weapon, but with minimal damage output.
The Sentinels of Onyx had significantly more powerful weaponry. They were described to be a sphere surrounded by three floating "booms", or small pieces of armor that possessed powerful energy shields which would suddenly "pop" into place in order to deflect objects moving at high velocities. Their shields are not activated by slow-moving objects, however - Team Saber used this to their advantage to destroy one of the Sentinels with rocks. Their energy weapons are described as a single, slow-charging, golden beam that could melt straight through the SPI armor of the Spartan-IIIs. SPARTAN-058 had also noted that one blast from these weapons was enough to drain the shields of her MJOLNIR Mk V armor. The Onyx Sentinels also have the ability to combine for different purposes, such as exponentially increasing their combat capabilities, or for large-scale excavation. One formation of these combined Sentinels was able to easily destroy two Covenant destroyers. However, it should be noted that the second Covenant Destroyer was caught off guard after a Slipspace jump, and the first one had its energy shields down.
The Forerunners also constructed Automated Turrets that fire blue beams similar to those of sentinel majors. These turrets themselves resemble stripped-down sentinels, and hover in mid-air, tracking enemy targets, but are otherwise immobile.
In Halo 3, it seemed that even the Monitors had the ability to defend themselves against the Flood using a focused beam similar to the Sentinel's beam, but much more powerful. 343 Guilty Spark displayed this ability in Floodgate and Halo. The weapon was capable of draining the shields of a Spartan-II in one blast and mortally wounding Sergeant Major Avery Johnson. It is also quite odd and puzzling as to why 2401 Penitent Tangent didn't use this ability against the Gravemind in Halo 2. It is assumed that the Gravemind is likely to have damaged 2401 Penitent Tangent when he captured the Monitor, disabling its beam weaponry. It could simply be that it was not equipped with one, although it is possible the Monitor attacked the Gravemind every chance he got, explaining his red "eye".
Forerunner architecture is noted mainly for being large-scale and geometric in style, with a triangular theme being the most commonly observed design. The Forerunners had a habit of creating natural habitats, such as those of the Halo rings, which tend to be very elaborate, with numerous extrapolations of the basic design of a construct appearing to be purely decorative. In addition, the structures were engineered to compliment the natural landscape, as opposed to the modern idea of replacing it. That said, their distinctive beauty are not restricted to Halo Array network. The Aztec-esque stones of Cote D'Azur, the arches and weathered inscriptions of Sigma Octanus IV, the intricate caverns and three-kilometer holographic dome beneath ONI's Reach complex, the ancient stepping-stones of the Grunt's homeworld, and the Forerunner City discovered in the depths of the human-inhabited Onyx, all display the Forerunner's innovative architectures. Most Forerunner architecture is constructed with a special type of metal that resists deterioration, bullets, plasma bolts and fire, as evidenced by Forerunner structures standing in pristine condition 100,000-150,500 years after they were built. However their resistance to plasma fire is limited; in Halo: Ghosts of Onyx the Elites blasted a hole in a Forerunner wall with plasma weapons. In Halo 2, Sgt. Avery Johnson destroyed the door to the control room of Delta Halo with a Scarab. Other structures are constructed out of traditional materials, such as chalcedonic quartz, which does not preserve nearly so well. These structures employ complex geometric shapes that appear to operate in dimensions higher than human understanding can comprehend. The Forerunners also decorated the interiors of their structures with a complex web of engraved straight lines and applied decorative touches and designs to nearly everything that they built, from structures to weapons. Even the Sentinel robotic drones possess small holographic Forerunner script around their "eyes".
Forerunner architecture distinguishes itself from Covenant structures in that they incorporate the use of angles, usually at either extremely sharp degrees (as the Forerunner buildings are usually triangular), or at forty-five degree angles, most notably seen in Halo: Combat Evolved . Originally, the ideas and concepts on Forerunner structures and technology was that it would be sleek and curved to give it an advanced feel. However, Bungie wanted the structures to feel as if they could stand the test of time and redesigned them to be solid and monolithic. The smooth, near-organic design would later be adopted by the Covenant in their architecture, although they did not adopt the Forerunner tradition of decorating their structures with glyphs. In Halo Wars the Forerunner counting system is used to open and close portals on the last level.
While the Forerunners' architectural and technological prowess is legendary among the Covenant and UNSC, their masterpieces also extended to the fields of stellar engineering. At least three Forerunner sites, the Shield World Onyx, the Ark, and the Micro Dyson Sphere that holds the Apex, made use of suns where none should exist. The Shield World existed permanently in Slipspace, where solar systems outside of a Dyson Sphere cannot exist, and the Ark was located far outside of the galaxy where galactic material would have been insufficient to form the sun that illuminates the Ark. The sun at the center of the Dyson Sphere where the Apex was housed was abnormally small, in order to accommodate the planet-sized Dyson Sphere. Whether these suns are artificially engineered or simply moved from another star system by the Forerunner is unknown: either would have been an enormous technological achievement. Another known Shield World is the planet discovered by the Spirit of Fire. This world had a outer crust like Onyx, but its inner Dyson sphere was accessed through a long narrow tunnel hidden under one of the planets oceans. Once through the tunnel, the Spirit of Fire had to pass through several "cleansing rings" in order to make it to the center of the planet. This Dyson sphere, unlike the Onyx Dyson sphere, was not separated in slipspace but was actually constructed on what appeared to be hollowed out inside of the planet. This sphere seemed to have had an artificially created sun like the one on the Ark.
The Ark is confirmed to be artificial, due to its unnatural "flower head" shape. It has been theorized to be an Alderson Disk. It is likewise unknown if these are the only stellar engineering achievements the Forerunners made. However, it is clear that manipulation of slipspace allowed the Forerunners to transport and position these structures throughout the galaxy.
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The Forerunner glyphs seem to be based on a series of circular, complex shapes. The glyphs have been inscribed almost everywhere, from different areas of Earth, to the Halos.
The Covenant venerate the Forerunners as gods and referred to them as the Ancients, believing that the Halo Array elevated the Forerunner to godhood instead of killing them. As such, they have appropriated many Forerunner technologies and artifacts, and such searching led to the Covenant coming to Harvest in the first place, therefore beginning the Human-Covenant war. Also they dedicated a significant amount of time to the search for other "holy relics" such as the Halo Installations, believing that they can follow the Forerunners into godhood. This is referred to as the "Great Journey" by the Covenant races. The Covenant believe that the Forerunner's ascension to godhood was finalized after the rings were fired.
The Control Module for the security system of the Silent Cartographer.
A Holographic representation of Installation 04.
Found in the Halo 3 Iris servers, these symbols are possibly part of a Forerunner numbering system. If the above image is correct, it is speculated that the Forerunners had a base six counting system unlike our base ten system. These symbols also appear on the last level of Halo Wars.
A Forerunner activating a console.
|Battle of LP 656-38 e • Battle of DM-3-1123 b • Battle of CE-10-2165-d • Battle of G617 g1 • Battle of the Maginot Sphere • Battle of the Halo Array|
|Librarian • Didact • 032 Mendicant Bias • Offensive Bias • Gravemind|
|Terminals • Keyship • The Ark • Maginot Sphere|
|Primary Pioneer Group • Advance Survey Team-Alpha • Emergency Circumstance Fleet • Suppression Fleet • Security Fleet • Forerunner Fleet Command|
|[SOURCE] - [EDIT]|
|· United Nations Space Command ·
· Covenant Empire · Forerunners · Flood ·
· Loyalists · Separatists ·
· United Rebel Front ·