The life of the Didact
The Didact was once the supreme commander of the entire Forerunner military, he was a Military genius, and proficient in the art of war, he was unmatched in combat, and a sight to behold standing over 14 feet tall, like all Forerunner wore a Hyper advanced exo-suit of armour. The Didact was opposed to the creation of the Halo rings he saw them as a sin above all else, that they stood against the very fabric of the Mantle (The Forerunners belief that all life is equally precious, and that if possible it must be preserved above all else) this was the core to the Forerunner belief system, war, to the Forerunner was looked at as a last possible resort, being looked down on, and seen as shameful in a way, it was only to be done if all other options were exhausted.
The Didact was the lover of the Librarian, the highest ranking of the Lifeworkers (A social class the specialised in all forms of life within the Forerunner), she was responsible for the preservation of all life before the Halo's were activated, she hid every species she could gather safe inside the Halo rings. Eventually they were sent back to their worlds of origin to re-evolve and thrive.
Before the Flood were a direct threat to the Forerunner many hundreds of thousands of years ago they were a threat to the Humans, and San 'Shyuum (the Prophets), this eventually took form of a losing war, and when the humans eventually started losing planets, they were forced to push their colonies back into Forerunner territory, eventually forcefully taking it. When all looked lost, the humans found a cure for the Flood and beat them back, but the cost was great, and before this the Forerunner had been forced to defend themselves against the humans forceful entering, and taking their planets, the humans were now at war on two fronts, unbeknown to the Forerunners at the time. Unfortunately once the Forerunner war machine was started there was simply no stopping it, the humans and Forerunners war had a long and brutal war, it was the Didact who controlled the Forerunner military at this time, and it was he who defeated us. Once the Creation of the Halo rings started, the Didact was so opposed to them that he chose to exile himself rather than take part in their creation. He hid on Charum Hakkor (Ancient earth) inside his Cryptum hoping not to be found. Afterwards the Humans and the San 'Shyuum (Our allies at the time) would both be put onto separate planets, our ancient home worlds, and devolved back to primitives by the Halo rings as punishment. About a thousand years afterwards a young Manipular (A Forerunner builder class) by the name of Bornstellar Makes Eternal Lasting, and and two humans found the Didacts Cryptum, while looking for treasure, soon after re-awaking the Didact.
Eventually the Didact melded with Bornstellar, giving him his memories, knowledge, and many of his physical traits, eventually being re-embodied as a new form. He would become a second Didact.
The Precursors and eventual revenge
The Precursors are believed to be the creators of most life in the universe. Advanced beyond measure even to the Forerunners, they were looked to as Gods by them. Not much is known about them other than they disappeared long ago. It is now thought that the Precursors created the Forerunner as their intended inheritors, though the Precursors eventually deemed them unworthy, choosing the Humans instead as their heirs. Whether this is true or the Forerunners simply betrayed the Precursors is not fully known. What is known is that the Forerunners went to brutal war with the Precursors and eventually wiped them out. A creature known at the Primordial, once thought to be the last surviving Precursor, (which was later revealed to be a hyper evolved Flood Gravemind) told the Didact, "We meet again, young one. I am the last of those that gave you breath and shape and form, millions of years ago. I am the last of those your kind rose up against and ruthlessly destroyed. I am the last Precursor. And our answer is at hand." This implies that the Flood were the Precursors' last revenge on the Forerunners for their betrayal.
Now in Halo 4 the Didact sleeps inside Requiem, once again inside his Cryptum waiting to be awakened, for trying to use the Composer to digitize humans. He was sealed by the Librarian.