John-117: Who are you?
Librarian: I am what remains of the Forerunner, once known as the Librarian. My memories were retained to assist Humanity on their path to the Mantle. Though sadly, that plan is now at risk. The Didact is leaving Requiem - soon. You must not allow it.
John-117: Leaving?
Librarian: He seeks this - the Composer. A device that will allow him to finally contain the greatest enemy ever faced by the Forerunners. You. Mankind spread into the stars with an unexpected, desperate violence. Entire systems fell before the Didact's Warrior Servants rose up to halt the aggression. When the Didact finally exhausted the Humans after a millenia, his sentence was severe. We had no way of knowing that the Forerunners were not your only enemy. Humanity hadn't been expanding. They were running. Weakened from our conflict, we were no match for the parasite which pursued you. The Forerunners made plans for a final, great yourney. But the Didact refused to yield our Mantle of Responsibility. He would save all life in the Galaxy... at a cost. In the Forerunners' quest for transcendence, the Composer had been intended to bridge the organic and digital realms. It would have made us immortal. But its results soured. The stored personalities fragmented, and our attempts to return them to biological state created only abominations. Such moral concerns faded from the Didact's attention. The Flood only assimilated living tissue. The Composer would provide the Didact his solution... and his revenge.
John-117: The Prometheans... they're Human-
Librarian: They were only the beginning. He would have encrypted your entire race if we had not removed the Composer from his care and imprisoned him here. Reclaimer, when I indexed mankind for repopulation, I hid seeds from the Didact. Seeds which would lead to an eventuality. Your physical evolution. Your combat skin. Even your ancilla, Cortana. You are the culmination of a thousand lifetimes of planning.
John-117: Planning for what?
Humanity, as decided by the Precursors, was meant to take over as the Caretakers of the Mantle. The Forerunners, who were at the time the current Caretakers, had squandered their responsibility, and upon finding out that they had been chosen against, they rebelled against the Precursors and in effect wiped most of them out. Those who survived fled the galaxy, only to return later, and quite subversivly, as what would become the Flood.
Later, the Forerunners, after having pushed back the Precursors, set out to war against Humanity in order to de-evolve their entire race so they could not then take the Mantle in which they were supposed to obtain rightfully. What the Forerunners didn't realize was that Humanity's vast and unprecedented spread throughout the galaxy wasn't out of conquest, but out of cleansing. Humanity was attempting to eradicate the Flood before it could spread even further. There was a possible cure/solution to the Flood that Humanity knew and/or harbored, but that information was lost or hidden during the Human-Forerunner War, which Humanity eventually lost.
As the war ended, Didact and Librarian had two opposing views of what should happen to the Humans. Didact wanted to turn them into Prometheans. Librarian wanted to preserve them, to be saved and to be given another shot, later on, as she still held onto the view that Humanity is the true heir to the Mantle.
The final decision was that the Halo rings were to be used to wipe out the Flood, and to eradicate all life in the galaxy that was not kept safe on the Halo rings themselves as seeds. UrDidact was imprisoned by the Librarian on Requiem. IsoDidact, Chant-to-Green, and many other Forerunners departed the Milky Way. Librarian stayed behind to oversee the firing of the Halos.
On Genesongs:
Certain humans were seeded with genesongs (essentially: an implant of the conciousness of another being's thoughts and memories and personality). Chakas was seeded with the Lord of Admirals's geas. Riser was assumingly seeded with IsoDidact's geas. Vinnevra was seeded with Librarian's geas. These genesongs would then travel through the family trees of these indiviuals up unto certain points, when they would be awakened within the humans they were predestined to be imposed on in the first place. Chakas was composed into 343 GS, so his genesong was lost for a time, until it later was awoken after the events of Halo 3. Halsey is a descendent of Vinnevra, possibly, and her genesong was, in theory, passed to Cortana. Riser's genesong is, in theory, awaiting to be awoken within John 117.