The Forerunner race would not exist, were it not for the highly advanced race, called the Precursors. The Forerunner race blossomed on their homeworld Ghibalb, and it did not take long for them to develop a real technological society. But even a highly intelligent species like them were not immune to mistakes, as their technology grew faster than their understanding of them. When an experiment in astroengineering went horribly wrong when nearby stars went supernova, their beautiful, paradise planet turned into a bleak and uninhabitable place.
As the Forerunner colonised more and more worlds, their numbers grew accordingly. It was stated by both the Ur-Didact and the Iso-Didact that they had received the "Mantle of Responsibility," the center of their society and culture, from the Precursors. In this however, they were wrong. Unbeknownst by them, as it happened roughly ten million years before the birth of the Iso-Didact, there had been a war. The Forerunner-Precursor war had taken place, after the Forerunners were denied the "Mantle" as it was given to humanity. It is unknown how, but the Forerunners eventually defeated the Precursors, and killed most of them.
After the events of the Forerunner-Precursor war, and the destruction of their creators, the Forerunner race took up the Mantle and they would spread across the galaxy. For their new empire, called the Ecumene, they build their capital Maethrillian which would control the three million inhabited fertile worlds in the Milky Way galaxy. The Forerunners themselves were a race almost entirely without conflict, and at some point in their history reached a point of disarmament, believing they no longer needed weapons in their rule of the galaxy.
While the Forerunners were keeping the peace in the galaxy, an extragalactic event caused millions of glass canisters to be transported to the Milky Way galaxy. These canisters were filled with a strange powder, which seemed harmless to the ancient humans who found them. They did not know however, that this powder was in fact Precursor genetic material, and would mutate any living organisms it came into contact with. They had found the Flood.
Once the humans identified the threat, they fought not only for survival, but also to cleanse the galaxy from the threat. This did not go unnoticed by the Forerunners, as the humans also cleansed Forerunner planets. Not identifying the threat correctly, the Forerunners assumed humanity was expanding rapidly, and with force. Fighting the humans with what little weaponry they still had, the Forerunners defeated the humans, albeit with great losses. They would recover, but their punishment of the human race was severe. Still convinced that they were aggressively expanded their empire, destroyed everything what was left of the human empire and put devolved versions of humans on Erde-Tyrene. Here, they would have to evolve into a more peaceful race.
And then, the Flood showed itself. At first the Forerunners did not know what to make of it, as it showed no immediate threat. Instead of attacking and consuming the Forerunners, they were docile and seemingly more interested in the Forerunner technology. This turned out to be their strategy, as they were not only numerous, but also spread out. When the Flood attacked, they took out this first Forerunner planet with ease. As the Flood absorbed all of the Forerunner's knowledge present, it took control of the Forerunner ship that were present and used them to spread out across planets.
Now identifying the Flood as a substantial threat, the Forerunners were quick to make weaponry capable of battling the approaching Flood. There were many attempts of defeating the Flood, but none seemed effective. Even their method of forcing stars to go supernova, thus destroying star systems at a time, was insufficient in defeating the threat. Many Flood spores were captured in order to better understand the enemy. These attempts proved unsuccessful, as the Forerunners could not provide an answer to the question of how they could be beaten. The only thing that the researcher could say was that they would die out if there were not able to feed. This led several prominent Forerunner scientists to design a large network which would wipe out all life-forms in the galaxy on which the Flood could feed. This network of Halo Arrays would consist of 12 Installations in the galaxy, and two "Arks" from which to operate the entire system at once. To prevent an irreversible galaxy-wide extinction, the Forerunner Lifeworker Guild would capture and catalogue all species so they could be reseeded after the extinction event. In order to prevent the extinction of the Forerunner race, Shield Worlds were created to shelter in.
This was the plan, but the plan was not put into action for many thousands of years, due to the Didact. He and his followers saw this plan as the greatest possible betrayal of the Mantle of Responsibility, as it did not prevent the hurting of species, but caused it. With all his being, he fought against the construction, but after several millennia he finally lost the battle. In the aftermath, the Ur-Didact was reprimanded by the Council and he shut himself in a Cryptum. After the Halo Array Network had been build, the Iso-Didact took control of the system. Meanwhile, the Forerunner-Flood war had raged on, and the Flood had perverted the Forerunner Ancilla 032 Mendicant Bias which attacked the Forerunner capital, causing the destruction of 5 Halo Arrays and the Maginot sphere. In order to halt the rampant Ancilla, the Forerunners created Offensive Bias, which was specifically created to take advantage of Mendicant's rampant spikes. Accelerating their plans with the Halo Arrays, the Greater Ark was host of the final battle of the Forerunner-Flood war. The war ended in the firing of the Halo Arrays, and thus the destruction of the Flood, and the complete destruction of the Greater Ark. Mendicant Bias was disassembled by Offensive Bias and split into many 'shards', one of these shards eventually made it to High Charity where it was perceived as an Oracle. After the events on the Greater Ark, the Keyships on the Lesser Ark were sent on their way to reseed the galaxy.
The End, And Resurrection
The battle on the Greater Ark and the subsequent firing of the Halo Arrays were fatal to all sentient lifeforms which were not protected against them. This meant the end of the Forerunner race, as the Halo Array's firing was much sooner than planned, after Mendicant's attack on the capital. There has been only 1 known Forerunner who has survived this event, the Ur-Didact. During the last meeting between the Ur-Didact, Iso-Didact and the Librarian, the Ur-Didact stated that he percieved them to be traitors to the Forerunner race, and he would find his own way of fighting the Flood. This plan turned out to be to use the Composer to harvest Omega Halo's entire human population and turn them into Prometheans Knights and fight the Flood with them. In this he was successful, and he made his way to Requiem. He was following by the Librarian however, who attacked him and put him into another Cryptum. The Ur-Didact stayed in this Cryptum for thousands of years, when he was eventually freed by the human warrior John-117. Now free, he quickly tried to take revenge on humanity, but he eventually was stopped. He would be Composed, but not truly die.