Ur-Didact

"The peaceful one is at war without and within."

- The Mantle, Fifth Permutation of the Didact's Number

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. On Charum Hakkor, he met a Lifeworker named the Librarian and married her. They had several children, who followed their father's footsteps and became 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 Charum Hakkor campaign, 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 led 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. With the mutation, the Didact transferred his consciousness, along with all of his memories and knowledge to Bornstellar. After his capture at Janjur Qom, the Didact was executed at the hands of the Master Builder after refusing to divulge information on the shield worlds or the control codes for the Contender-class ancillas.

Reincarnation
After his original body had been killed, the Didact's personality imprint in Bornstellar's body took full control of the Manipular, effectively making him the reincarnation of the Didact. After surviving the assault by Mendicant Bias on the capital, the reincarnated Didact reunited with the Librarian on Installation 00 and resumed his duties as supreme commander of Forerunner military due to the present threat of the Flood.

Soon after, the Didact tracked down the Halo installation carrying Mendicant Bias and the Captive from Charum Hakkor, as well as the humans who had accompanied him after his revival. His fleet intercepted the Halo ring, on a pre-determined collision course with a planet in a distant system. He proceeded to deactivate Mendicant Bias using his control codes and save the Halo and its research data on the Flood.

After the Halo had been successfully transported to the greater Ark, the Didact, accompanied by a wounded Chakas, interrogated the Captive, now imprisoned once again. Over the course of the exchange, the entity revealed the true nature of the relationship between the Precursors, the Flood as well as the Forerunners and humans, stating that the Forerunners were never meant to inherit the Mantle and that there was no cure for the Flood. Furious, the Didact then executed the creature by fully activating its slipspace stasis chamber, forcing it to live through a billion years in complete sensory isolation and causing complete physical entropy.

Following his temporary victory, the Didact's power rose as he returned to the favor of the re-established Ecumene Council, also elevating the status of his wife. The debate on the Didact's plans for stategic solutions against the Flood was restored and the use of the Halos was delayed for a time.

Endgame
"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.

Later on, during the war, the Didact 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".

After the Didact learned the Librarian's location, he prepared a rescue party to retrieve her. However, before they could depart, Mendicant Bias' fleet breached the Maginot Line and destroyed the rescue party. Stricken by grief, the Didact then activated the Halo Array, wiping out all sentient life in the galaxy.

Trivia

 * A centuries-long series of conversations between Didact and the Librarian can be found inside Forerunner terminals in Halo 3. When the terminals are accessed, the player is eventually moved to a conversation that was recorded before the Halo rings were fired but after he had transferred his consciousness into Bornstellar. The Didact's last words to the Librarian moments prior to the Array's activation are featured in the Iris campaign Server Episode 1.
 * The title Didact is derived from the word didactic, which means to instruct others, especially with regard to morals. This is ironic because the Didact, who sought the dismantling of the Halo array to protect the galaxy, ultimately activated the rings, defying his morals.

List of appearances

 * Halo 3
 * Halo: Combat Evolved Anniversary
 * Terminals
 * Halo: Evolutions - Essential Tales of the Halo Universe
 * Soma the Painter
 * Forerunner Saga
 * Halo: Cryptum
 * Halo: Primordium
 * Halo Legends
 * Origins
 * Iris
 * Halo 4