Ur-Didact

"Mendicant Bias is trying to prevent us from firing the Array. He speeds back to the Ark, but he won't succeed. Offensive Bias will stop him, and I will burn this stinking menace in your name. And then? I will begin our Great Journey without you, carrying this bitter record. Those who came after will know what we bought with this [false transcendence] - what you bought, and the price you paid."

- The Didact's final transmission.

The Didact was a Forerunner Promethean who held an extremely high status in the Forerunner society as supreme commander of the entire Forerunner military. He evidently wholeheartedly believed in the "Mantle" the Forerunners held to protect life. He was also the lover of the Librarian, and the one who eventually activated the Halo Array.

Biography
The Didact was a Promethean, a member of an extremely powerful class of Warrior-Servant. At some point and for unknown reasons, he entered a state of meditation inside a Cryptum, which somehow ended up in Djamonkin Crater on Earth. Over 10,000 years later, he was discovered and awoken by a young Forerunner Manipular, Bornstellar Makes Eternal Lasting, who had come to Earth in search of Precursor relics.

During his subsequent travels with Bornstellar, the Didact performed a brevet mutation on him to allow him access to the Domain. Shortly afterward, the Didact transferred all of his memories and knowledge to Bornstellar, essentially merging his personality with the young Forerunner. The Didact was later executed at the hands of the Master Builder, making Bornstellar, for all intents and purposes, the reincarnation of the Didact. He would go on to meet his wife, the Librarian, aboard Installation 00.

Forerunner-Flood War
During the Forerunner-Flood War, he was constantly trying to convince the Librarian to give up on her mission to save other sentient life from the Halos, and return to him "Where it is safe". His pleas, however, were unsuccessful, and the Librarian ended up destroying her own fleet, stranding herself on Earth to live out the rest of her days in a place she referred to as "Eden".

Fate
Didact's eventual fate is unclear, whether or not he survived the activation or perished, as his location is not specified, be it on the Ark, or one of the Halo Rings.

While the Server's first episode seems to state that Didact perished, though Terminal 7 seems to contradict this, as Didact describes going on the Forerunner's "Great Journey" without the Librarian, carrying the "bitter record" of the Forerunner's failure. Whether this describes an actual physical event and his survival, or a more ideological/religious concept involving the Forerunner's concept of death and the afterlife that he is describing, is unclear.

Trivia

 * A conversation between Didact and the Librarian can be found inside the Terminals in Halo 3. When the Terminals are accessed, the player is eventually moved to their old conversation that was recorded before the Halo rings were fired.
 * In the Iris campaign Server Episode 1, Didact's last words to the Librarian can be read moments prior to the Array's activation.
 * The word "Didact" actually means "one who teaches" in Greek. From this information, one could infer that at one point in his life he did some important teaching in Forerunner culture. The name Didact could come from the word didactic which means to instruct others in morals. This is rather ironic because he wound up being the one committing galactic genocide, firing the metaphorical gun at the head of the galaxy.
 * It is never made clear in the terminals if Didact is a name or a title, or whether the two are interchangeable.

List of appearances

 * Halo 3
 * Forerunner Saga
 * Halo: Cryptum
 * Halo Legends
 * Origins
 * Halo: Evolutions - Essential Tales of the Halo Universe
 * Soma the Painter
 * Iris