Planet of Origin: Ghibalb, a planet located in the outer edges of the Orion complex. Once a beautiful, verdant paradise of a planet, it was cleansed of all life when an experiment in stellar engineering accidentally caused several nearby stars to supernova. It is now heavily irradiated and uninhabitable.
The Forerunners are a sentient species that developed an advanced FTL interstellar civilisation and praticed world building. They are responsible for construction of the 2 Arks structures that are used to ringworld-type fortress (from 10 000km to 30 000km in diameters) that formed the Halo array around the galaxy. The Array was designed to kill all sentient life in the galaxy which was not protected inside specially designed Dyson-type shield worlds and therefore rid the Milky Way of the Flood parasitic lifeform.
While not record of the Forerunner's appearance remains, their battle armors show them to have a humanoid shape disturbingly close to the human form.
A very advanced civilization, they came to power after the destruction of another race, known to them as the Precursors. The Forerunner believed that the Precursors were responsible for their creation. Soon after, the Forerunners became the dominant species in the galaxy, their empire spanning over three million fertilized and inhabited worlds. The Forerunners reached their peak before activating the Halo Array around 100,000 years ago in the closing days of the Forerunner-Flood war.
Their civilization was based around the Mantle, a belief they inherited this belief from the Precursors that it was their role to protect all life in the galaxy. They presumably monitored the evolution of other sentient species throughout the galaxy, intervening in ways that ensured their civilizations would follow paths of peace, free of conflict. One such occasion is the devolution of the prehistoric human empire officially to prevent further conflict emanating from what they considered a "violent species" some 110 000 years ago, but officiously to prevent them from taking their rightful place as the Precursor's heirs to the Mantle. After their superiority was insured, the Forerunners reached a point of disarmament, believing they no longer needed weapons in their rule of the galaxy, which ultimately weakened them when the Flood attacked.
When the Forerunners were unable to contain the Flood outbreak, the galaxy entered a state of war. However, all their tactics, were they to turn star novae to eradicate infected systems, proved only to slow, but never stop the expansion of the Flood. They realized that the only way to stop the Flood was to deprive it of any and all hosts, thus eliminating its potential to grow. Eventually, the Forerunners 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. 100 000 years ago, after having exhausted every alternative, they ctivated the Halo array, killing themselves and all sentient life of sufficient biomass in the Milky Way, with the exception of those species safely placed on the Ark. After the Array was fired and the Flood had been eliminated, the Forerunners left behind an automated system of automatons, much like the Sentinels, and AIs to spearhead the reseeding of the galaxy with those data logs, embryos, and specimens contained on the Ark.
The Forerunners are adored like gods by the alien league known as the Covenant. The fact that the Forerunners appeared to be genetically related to the humans explains why the Covenant seeks to exterminate humanity, knowing that they are the Forerunner's designated heirs and would undermine their power.