The Forerunners
Forerunner is the translated name for the ancient race of enlightened beings that were the creators and builders of the Halo Array, the Arks, and the Shield Worlds, along with the artifacts that were found on the planets Reach, Earth, Sigma Octanus IV, Harvest, Arcadia, Sanghelios, Kholo, and Janjur Qom. 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 mutilated or destroyed these relics to be heretics. The Forerunners themselves were a bipedal species with five-digited hands, but this may have been Cortana's interpretation of the evidence.
The Forerunners were a very advanced civilization, coming 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, such as similar physical appearance. 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 prior to 2552, in the closing days of the Forerunner-Flood war.
Being one of two known races ever to achieve Tier 1 technology (the other being Prehistoric Humanity) based on the Technological Achievement Tiers measurement, they were only superseded by the mysterious race of beings called the Precursors at Tier 0, evidence of whom have been found on various planets.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. Their civilization was based around the Mantle, a belief 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 human empire to prevent further conflict emanating from what they considered a "violent species". The Forerunners themselves were a race almost entirely without conflict, and at some point in their history had 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.
Later in their history, a Forerunner survey team came into contact with an extragalactic parasite known as the Flood on the planet G 617 g (details of this first contact are unavailable). After this first contact, the Forerunner immediately recognized the great danger that the Flood posed to the entire galaxy and attempted to contain them. However, even they underestimated the potential of the Flood to learn and adapt, believing it to be a non-sentient but highly contagious disease. As such, the attempt to contain the Flood was a failure, as they tried methods more suited to disease control than warfare, trying to contain and quarantine the Flood rather than attack and destroy it outright. They did not start war until it was far too late.
When the Forerunners were unable to contain the Flood outbreak, the galaxy entered a state of war. During the 300 years of the conflict, the Forerunner studied the Flood in labs such as the gas mine located in the atmosphere of Threshold, looking for any exploitable weakness. As the Forerunners soon realized that conventional naval tactics were ineffective against the parasite's onslaught, they developed new weapons and tactics to combat the Flood. The Sentinels were deployed as a means of maintaining the Flood through surgical, localized tactics. The Forerunner fleet command even contemplated using naval battle groups to enact premature stellar collapse within compromised planetary systems, causing supernovae to engulf entire worlds. However, as these tactics proved only to slow, but never stop the expansion of the Flood, the Forerunner leadership 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.
At first, the Forerunners were reluctant to use the Array, believing that they should continue to embrace the Mantle and to protect life rather than to destroy it. This led to great stretches of anguished debate and even civil war, but eventually it was decided that the Array was the only means of successfully stopping the Flood threat. The Gravemind managed to convince a Contender-class AI, 05-032 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 were forced to cede to the Flood. At last, the Forerunners exhausted every alternative and activated 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 sacrificed themselves in order to activate the array, their last military presence and members of their species defending the Ark until the Didact activated the Halo array, cleansing the galaxy.