Forerunner (Earth-5875)

"A hundred thousand years ago a great civilization existed in this galaxy. Like all great civilizations, they faced a sudden and dire turn of events. A threat to their primacy from the outside. Something they never expected, never prepared for. A deeply alien threat they called simply the xenomorphs, an unstoppable force. It devoured everything it touched. At first their technology and couraged seemed like it might prevailed. But they waited too long to see the threat, to join the fight. The xenomorphs had spread too far, and too wide. It fed on their life, and in doing so, it became ever more intelligence itself. The xenomorps were unique, using their very strength against them. They were the forerunners."

- - Cortana recounting the events of the ancient Forerunner-Xenomorph War The "forerunners" (Latin Mundus gubernavi, meaning "universal pilot"), also known as the Gods to numerous species, are an ancient sapient species which had since old times achieved advanced technology and interstellar travel. With the Mantle, the forerunners believed themselves to be the caretakers of the Milky Way galaxy and all of its wonders. Their empire, the Ecumene, existed for millions of years, acquiring dominance over the entire galaxy. After thousands of years, the extragalactic sapient species the forerunners named the xenomorphs emerged, threatening to destroy all life.

After entering into the millenia-old Forerunner-Xenomorph War, the forerunners attempted to exterminate the xenomorphs with all of their weapons and technology, but to no avail. They then resorted to a last, desperate solution, the creation of the Halo Array, which exterminated all life in the galaxy, including that of the xenomorphs and their own. Evidence of their civilization and remnants of their technology continued to exist, with some organizations, such as the dissolved Covenant, believing the forerunners to be gods.

Origins
Little is known about the orokin's history other than the existence of their ancient empire, the Ecumene which encompassed most of the Milky Way galaxy. Their society was divided in rates, castes determined since birth to each forerunners, these being the Warrior-Servants, dedicated to warfare, Builders, dedicated to engineering, and the Lifeworkers, dedicated to science and research. They are the creators of the huragok, a sapient nanotechnological sapient species, who helped them maintain their civilization. The forerunners possess an unknown and mysterious relationship with humanity, the latter of which they share numerous physical similarities, including five fingers, one opposable thumb, and humanoid form, having visited Erde-Tyrene in the past, where a lone Sacrificial Lifeshaper sacrificed his own life by ingesting the black liquid, causing his body to dissolve and seed the primordial planet with his DNA. The forerunners secretly continued to visit Erde-Tyrene for the next hundreds of years, observing and studying humanity and wondering about their similarities. The orokin lived by the philosophy of the Mantle, which was guardianship over life, in a society best described as an utopia, possessing Tier 1 technology, only surpassed by the mythic precursors, which they believed were their creators.

Forerunner-Xenomorph War
The orokin's reign through the galaxy became threatened when, in 97,745 BCE, they came in contact with the xenomorph, an extraterrestrial sapient endoparasitoid species from outside the galaxy, who landed on G 617 g, one of their frontier worlds near the galactic center and started to infect and kill all life they came across. Despite their advanced technology, the orokin had never come across something as different and deadly as the flood, whose power far exceeded their own. This led to the catastrhopic, millenia old Forerunner-Xenomorph War, which saw most of the life in the Milky Way being extinguished by the extragalactic threat. The xenomorphs consumed forerunner worlds one by one, ever expanding into numerous forms, such as hundreds of Keyminds and subsequently, Graveminds. The forerunners attempted to contain the xenomorphs through numerous forms, all which failed.

Firing of the Halo Array and extinction
They eventually decided that the only way to kill the xenomorphs was to kill their own source of spread, including all remaining living species in the galaxy and themselves, and for that they built the Halo Array and the Ark, superweapons that, when activated, would release an incredibly strong radius of power that would purge all life within the Milky Way. After building the seven structures for two years, they finally released them, assigning one Monitor to each installation, that would caretake and secure them. Before the activation of the rings, the Ecumene made humanity, their "children", Reclaimers of everything the orokin left behind, being the only ones able to access their more advanced installations.

After the Didact activated the Halo Array, he, alongside all forerunners, were slain by the pulse radius of the rings, and the species became virtually extinct, with the exception of the imprisoned Didact. Despite their departure, remnants of the forerunner civilization and technology remained scattered across the galaxy, through planets and star systems. The Librarian had been able to save the remaining sapient species of the galaxy and indexed them in the Ark, as a part of the Lifeworkers' Conservation Measure. After the Halos were fired, all species were returned to their homeworlds through automated Keyships. Flood specimens were indexed in each Halo installation, were they kept being studied by Monitors and Sentinels in an attempt to find a cure for their impregnation.