The Forerunners, their civilization being called the Ecumene, are an advanced alien species that existed before the activation of the Halo Array, their own creation. Their lifestyle is shrouded in mystery. With their disappearance from the galaxy 100,000 years ago, the only records left behind about Forerunner society is the scattered relics, ruins, and artifacts. What is known is that the Forerunners had a caste system composed of Builders, Miners, Lifeworkers, Warrior Servants, and Engineers (in that respective order). Despite the Builders being the highest rate in Forerunner society, the Ecumene Council dictated most Forerunnner affairs and policies.
Just before the Forerunner-Flood War began, there were approximately 22,691,100,000 Forerunners situated in the Orion Arm. This doesn't account for those elsewhere in the galaxy. After the war, that number reduced to about 60 Forerunners.
The Forerunners had an empire that spanned across the entire galaxy and their civilization had controlled more than half of the worlds in the Milky Way. Their technology was superior to that of the Covenant and Humankind. In fact, much of the Covenant's technological achievements and religion is based off of recovered Forerunner relics. The United Nations Space Command had even reverse-engineered this technology and integrated it into their own devices.
In their time, the Forerunners were encountered by an intergalactic parasitic disease known as the Flood. The Flood was intelligent and motivated with the single purpose of consuming all life in the galaxy. The parasite spread across hundreds of Forerunner worlds and infected trillions of Forerunners. The more the Flood spread, the stronger it became. It could absorb the memories and knowledge of its victims and use that to coordinate effective strategies against its prey. The parasite could even use the Forerunner's technology in combat.
05-032 Mendicant Bias was the strongest artificial intelligence constructed in order to combat the Flood. Despite its impressive intellect, it fell victim to the logic plague that the Flood used to overpower AI constructs. Mendicant Bias suggests that it had not succumb to the Flood and instead infiltrated the Flood. Bias made the Flood stronger so it would persuade its makers to activate the Halo Array which was the only way to stop the Flood.
While the Flood pressured the Forerunners, their greatest military commanders watched from the Ark and coordinated the ongoing battle at the Maginot Line. This Line was established as a defensive perimeter to protect the remaining worlds in Forerunner space. Mendicant Bias had destroyed the Line, but when he did, the Forerunners had already activated the Halo Array.
The Logician was assigned as an overseer to the Halo Array. He would remain on the rings and assist their monitors; however, he spent most of his time on Installation 01 conducting experiments to stop the Flood. Installation 04 is one of the rings that was left abandoned by the Logician. Fortunately, the monitors were able to care for these rings on their own. Delta Halo hadn't been so lucky though, because it had outbreak issues and maintenance problems.
In 753 B.C.E., the Chulorg made contact with Installation 01 and settled on the ring along with the Logician. The Logician was viewed as a wise and knowledgeable God. In approximately 150 A.D., the Logician abandons the Chulorg and Beta Halo to investigate a signal he received from the Elder Codex.
In his absence and the absence of the Forerunner from the galaxy, civilizations across the galaxy were profiting from Forerunner technology left behind. Eventually, the humans and the Covenant came across the Halo Array. Installation 04 was destroyed to prevent the activation of the ring and spread of the Flood. Installation 05 was discovered and glassed when the Flood outbreak began. The Ark was even destroyed to prevent the activation of the entire array.
When the Logician noticed the destruction of these installations, he returned to Installation 01 to discover humans and Chulorg fighting over the ring. The Logician threatened to destroy the two factions. Putting aside their differences, the humans and Chulorg formed an alliance and defeated the Logician.
The Forerunners have a technologically advanced civilization that is capable of constructing or terraforming entire worlds, building weapons of mass destruction, developing teleportation technology, and constructing robotic and artificial intelligences. Even among their kind, the Halo Array is considered one of their greatest and horrifying achievements.
The Halo Array is a network of seven ring worlds scattered throughout the galaxy and acting as weapons that can obliterate all life in the galaxy once activated from the Ark. The Halo Installations have their own environments and structures designed to house life forms during the Conservation Measure. However, the rings are capable of generating a pulse that can destroy organic matter. When the rings are activated simultaneously at the Ark, their pulse radius can cover the entire galaxy, destroying all life and potential Flood food sources. The weapons were designed to starve the Flood. While the galaxy is being cleansed, the Ark and its inhabitants survive from outside the destruction range. Shield Worlds were designed to safeguard life from inside a Slipspace bubble that will protect all from the effects of the array. Installation 01 is distinguished from the other rings because it also serves as a testing ground for ongoing experiments to eliminate the Flood or find a cure should the parasite return. The Logician was designated as the Overseer of the Halo Array; however, he left most of his responsibilities to the designated monitors while he focused on the experimentations on Installation 01.
The Forerunners have settled on hundreds of worlds and left behind numerous artifacts and technology. Helena was one of their fortress worlds dedicated to their military effort against the Flood. There is a military base on the world; however, it was left guarded and protected from outside visitors. When the UNSC colonized the world, their people were unable to access the fortress because of an invisible dome shield. The fortress has a teleportation portal that connects to a weapons cache on Coral. The portal is no longer accessible as of 2554 due to the destruction of the cache. There are other structures on Helena, but they don't appear to have as great of significance as the main fortress.
Coral Weapons Cache
Coral is a distant world that was on end used by the Forerunners presumably as a weapons locker. There is one known structure on the planet that runs deep underground. It contains weapons on end used by the Forerunners during the Flood war. These weapons were studied by the New Colonial Alliance and used to manufacture laser-based weapons. The structure had a teleportation system with portals that connected the Coral to other installations such as the Ark Portal and Helena.
The Catalyst is a Forerunner petrification device that turns organic matter into a substance that can fuel their starships and other mechanisms. The Forerunner miners had used this artifact and others like it to turn plant life and deceased animals into fuel for their ships. Because of their beliefs, it was never used on living organisms, with the exception to plants. The Covenant had discovered the Catalyst and it eventually fell into the Jiralhanae's hands. The Catalyst was brought to Roost after the war, and it was stolen by Martin Stone, a cyborg soldier working for the Oversight Committee of the Office of Naval Intelligence. Chairman Ian Rivera saw the artifact's potential as a weapon and sold it to the Nova Adonis whom he believed would use it against the Sangheili and other former Covenant species.
The Venedico Artifact is a component used in Forerunner Slipspace drives that is necessary for opening the portal into Slipstream Space. The Covenant discovered this artifact on Venedico a couple of years after glassing the planet. Codenamed: CONDUCTOR learned that the Covenant had found something profound on Venedico, and Spartan-II Silver Team was sent to recover the device in Operation: VERTIGO. The mission was a success, and the device was brought to Algolis to be studied. This technology was reverse engineered and integrated into the UNSC Harbinger as a prototype. It was be further integrated into other starships following the Human-Covenant War and would allow the humankind to travel through Slipspace at faster speeds.
Bliss Collection Field
In 2557, the BXR Mining Corporation uncovered an alien structure while mining the silicate glass from Bliss. The structure was beneath their mining facility and it spanned 125 kilometers in diameter, approximately the same size as the Ark Portal. The UNSC uncovered the rest of the artifact and examined it. One of their soldiers had accidentally activated the machine and it shot a power beam towards the planet's local star. The remnant faction of the Office of Naval Intelligence had learned of this through their sources and sent agents masked as BXR employees to infiltrate the structure and turn off the device. ONI managed to shutdown the machine and gather intelligence on the artifact. Resulted from the event, Bliss' star was reduced in size and it was predicted that this would have a negative effect on the already glassed planet.