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 wholeheartedly believed in the "Mantle" the Forerunners held to protect life, and initially opposed the Halo Array as a sin beyond measure. He was also the lover and husband of the Librarian, and the one who eventually activated the Halo Array. Although he was physically killed by the former Master Builder Faber, he lived on by implanting his consciousness within a young Builder known as Bornstellar Makes Eternal Lasting.

Warrior-Servant career
The Didact was a Promethean, a member of an extremely powerful class of Warrior-Servant. He gained his name while teaching at the College of Strategic Defense of the Mantle. He eventually met a Lifeworker named the Librarian and married her. They had several children, who followed their father's footsteps at Warrior-Servants. During the Human-Forerunner War, the Didact lead the Forerunner military against the humans. Though all of his children died, the Forerunners were victorious. After the Battle of Charum Hakkor, he and a group of Prometheans, including the Confirmer, discovered a stasis capsule on Charum Hakkor which contained the Timeless One, the last of the Precursors. Following the end of the War and the first discovery of the Flood, he advocated a policy of vigilance and research in the event of a Flood resurgence. He also proposed the construction of Shield Worlds across the galaxy, in order to monitor the galaxy for Flood outbreaks and provide military support should they occur. However, a faction of Builders lead by Faber proposed the construction of a series of superweapons to deal with the Flood instead, and they were successful in convincing the Council to sanction their plans. Having lost the political battle, the Didact was stripped of his power and forced into exile.

Discovery on Erde-Tyrene
In his exile, the Didact entered a state of meditation inside of a Cryptum, which was placed by the Librarian in Djamonkin Crater on Earth. Over one thousand years later, he was discovered and revived 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 the young manipular 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 reunite with his wife, the Librarian, aboard Installation 00.

Forerunner-Flood War
During the Forerunner-Flood War, after his merging with Bornstellar, he was constantly trying to convince the Librarian to give up on her mission to save other sentient life from the Halo Array's inevitable activation, 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".

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. It is unknown if the conversation occurred before or after the Didact transferred his personality to Bornstellar.
 * 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 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.

List of appearances

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