The Didact

""So fades the great harvest of my betrayal. Even these beasts recognized what you were oblivious to, human. Your nobility has blinded you, as ever. The Librarian left little to chance, didn't she? Turning my own guardians... my own world... against me. But what hubris to believe she could protect her pets from me forever. If you haven't mastered even these primitives, then Man has not attained the Mantle. Your ascendance may yet be prevented. Time was your ally, human, but now it has abandoned you. The Forerunners... have returned. This tomb… is now yours." - Introductory speech to John-117 upon waking up from his Cryptum on Requiem."

The Didact, also known as Ur-Didact or the Shadow-of-Sundered-Star, is a Forerunner and the main antagonist in Halo 4. He is also the leader and creator of the Prometheans

Biography
The Didact was once the supreme commander of the entire Forerunner military, he was a known military genius, and was strategic and skilled in the art of war. He was unmatched in combat, and a sight to behold standing over 11.4 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 also 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, 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 Didact created the composer and got a new form, which disfigured him. He used the composer on his Prometheans. He blamed the Humans for the war and used the composer on them at Omega Halo. He was banished to Requiem for his actions. He was sealed in a Cryptum until he was opened by John and Cortana in 2557.

He then took control of the Rogue Covenant forces in Requim and started activating the Composer in order to attack Requiem with them. After he escaped the planet with his combined Covenant and Promethean forces, he started his attack on earth. However, he was pursued by John and Cortana and was stopped by them. John destroyed both presumably him and the weapon using a nuclear device. This resulted in Cortana sacrificing herself as well. Its unknown what happened to the Didact afterwards.