The Forerunner are shrouded in mystery. Although their cyclopean architecture still remains, they have long been absent from their own creations. What little we know of them is explained vicariously through their machinery and their automatons. Clearly, they were technologically superior to both Humanity and the Covenant. Their empire spanned at least this galaxy, encompassing both Einsteinian reality and Slipspace. They are not the Covenant's gods, but are revered as god-like, having supposedly found the path to salvation.
At one point in the Forerunner's history, they encountered the Flood. They fought, and retreated. When they had at last exhausted every tactical option, the Halo network was activated; as far as 343 Guilty Spark's data shows they, and all other sentient life in the galaxy, were all killed in the process. For whatever reason, they had felt it was necessary to contain and study the Flood, rather than destroy them outright. Their allowance of the Flood's perpetuation and storage in stasis could be considered an inexcusable lack of foresight for such an advanced civilization, though without knowing their precise reasoning, the passing of judgement is problematic.
The Forerunner perished (or fled) some 100 000 years ago, leaving their robotic compatriots behind to defend and monitor their installations. Seeing the Master Chief and Cortana breach protocol and refuse to activate Halo 04 upon the Flood's release, 343 Guilty Spark turned Halo's only known internal defensive system on them: the Sentinels. The Sentinels are floating Mantis-like machines, using what is presumably Forerunner weapon and shield technology. Unlike the the Human and Covenant arsenal, the Sentinels used some sort of controlled stream of energy. Deadly and precise, they appear to be more like surgical cutting tools than offensive weaponry, and while effective against the Flood's lower forms, are easily overcome by the Warrior class. Delta Halo 05 was also home to a series of smaller Repair class robots, as well as to the large Enforcer class guards.
Their relics are not restricted to Halo network. The Aztec-esque stones of C'ort Azur, the arches and weathered inscriptions of Sigma Octanus, the intricate caverns and 3km holographic dome beneath ONI's Reach complex, the ancient stepping-stones of the Grunt's homeworld, the Prophet's own claim to have evolved on an abandoned Forerunner planet, the revelation that their Ark (whatever that entails) exists on Earth, not to mention the countless installations undoubtedly pilfered to allow the Covenant to have achieved their current levels of technology, all give testament to this galaxy's permeation.
But who were these Forerunner; these Truth-Givers? What are the details of their relationship to the Covenant; what are they to Humanity? The Covenant, garbed in their glyphs, pay an eery homage to those symbols depicted in the interiors of Halo's buildings, whereas Humankind seems all too at home and recognized by the ring's Monitors. Much remains to be answered.
Une Histoire complete sur les Forerruners