Shadow-of-Sundered-Star

"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 of Responsibility" 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 abandoned in a Flood-infested system by the former Master Builder Faber, he lived on by implanting his consciousness within a young Builder known as Bornstellar Makes Eternal Lasting. He believed the Mantle of Responsibility should belong to the Forerunners alone and saw humanity as a threat rather than the successors that most other Forerunners saw them as.

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 lead by Faber proposed the construction of a series of superweapons to deal with the Flood instead, and they were successful in convincing the Forerunner 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 Bornstellar to allow him access to the Domain. With the mutation, the Didact transferred all of his memories and knowledge to Bornstellar. This made Bornstellar essentially a copy of the original Didact. The original Didact, or the Ur-Didact, was later left in a Flood-infested system by the Master Builder. After surviving the 05-032 Mendicant Bias' assault on the capital, Bornstellar Didact would reunite with his wife, the Librarian on Installation 00 and resume his duties as supreme commander of Forerunner military due to the present threat of the Flood.

Forerunner-Flood war
"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.

At some point during the Forerunner-Flood war, the Ur-Didact began to experiment on himself in an attempt to gain immunity to Flood infection. When his efforts ended in failure, a subordinate suggested the Composer as a solution. Since the Composer did not work on his new form, his Prometheans all volunteered to undergo the process, becoming the first Promethean Knights.

"Humans, your kind brought the Flood to our shores. Now you will aid in ending it's threat."

- Ur-Didact

Despite this new army of soldiers no longer susceptible to Flood infection, their numbers were too few to prevent the parasite from continuing to spread. Seeking to both increase the size of his army and exact a measure of revenge for their actions during the previous conflict, the Ur-Didact would turn the Composer upon humanity; planning to eventually have the entire species composed and turned into new Prometheans

After seeing the results of his "conscripting" one human village, the Librarian would remove the Composer from the Ur-Didact's custody and seal both him and his Knights away inside Requiem; putting an end to the Ur-Didact's plans and sentencing the galaxy to the activation of the Halo Array.

Towards the end of the war, the Bornstellar-Didact would constantly try 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; the Librarian destroyed her own fleet and stranded herself on Earth to live out the rest of her days in a place she referred to as "Eden".

Return
"Time was your ally, Human, but now it has abandoned you. The Forerunners have returned."

- The Ur-Didact to John-117, after his awakening.

In 2557, the Ur-Didact is awakened on Requiem when he tricked John-117 into releasing him by manipulating transmission signals from the, and leading him to the center of Requiem under the pretense that it was a transmission network when it was his Cryptum that sealed him within, and required a Reclaimer to activate it so he could leave. Stating that Humans were not fit to uphold the Mantle of Responsibility, he calls the Librarian 'meddlesome' in attempting to protect humans. After he regains control of the Prometheans and

The Didact, with the Covenant Remnant now at his command, then launches an assault the grounded but UNSC forces, aided by John-117, are able to push back the assault and even drive the Didact's vessel away. The Didact then begins a search for "the Composer", a device that could turn a living being into digital form. Eventually, he learns it is located at Ivanoff Research Station orbiting Halo Installation 03. Gathering his Covenant and Promethean forces, he departs Requiem for Installation 03, unaware that Master Chief and Cortana are stowed aboard a Lich. After arriving at his destination and fighting the UNSC forces there, the Didact is able to obtain the Composer and fires it at Ivanoff Station, instantly killing all UNSC personnel aboard with the exception of the Master Chief who, due to the Librarian's genetic manipulation, was made immune to the effects of the Composer.

The Didact then departs for Earth to create a new Promethean army, but once again Master Chief follows him, this time aboard a Broadsword fighter hidden in Didact's ship. Upon arriving at Earth, the Didact is immediately attacked by a UNSC fleet led by Infinity. Coordinating with Infinity, Master Chief is able to make a hole in the Forerunner vessel's hull and board it.

Apparent Defeat
"So misguided. Humanity's imprisonment… is a kindness."

- The Didact's final words to John-117

Master Chief eventually fights his way to the Didact; however he is too late to prevent the Didact from firing the Composer at Earth, specifically at the city of New Phoenix, which is instantly depopulated. Master Chief and the Didact engage in a brief battle, the Didact then claims that using the Composer on humanity to turn them into Prometheans is in truth 'a kindness' to them for destroying his people in the past. At which point Cortana and several of her clones manage to manifest themselves from the Hardlight bridge that the Didact was on and bind him to it, restraining the Didact long enough for Chief to plant a Pulse Grenade on his chest. It detonates, wounding him and causing him to fall off the light bridge and into the slipspace portal below. Master Chief then destroys the Composer and the Didact's vessel with a Havok nuclear bomb as Cortana shields him from the blast with Hardlight, ultimately sacrificing herself.

Trivia

 * A centuries-long series of conversations between Bornstellar-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. Also, in the Iris campaign Server Episode 1, Didact's last words to the Librarian moments prior to the Array's activation can be read.
 * 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.
 * Didact is the first Forerunner whose face is shown in the series.
 * Keith Szarabajka, also known for his performances as Dr. Kyne in Dead Space and Harbinger in Mass Effect 2, provides the voice for the Didact.
 * Shortly after his awakening in Halo 4, the Didact refers to the Sangheili nearby as primitives, while speaking to the Master Chief as a peer, if not an equal. This is curious, as the Sangheili are at a higher technology tier than humanity.
 * This can be explained one of two ways; either it is because when the Didact had last been awake, the Sangheili were a Tier 7 civilization or not even sapient. That, or humanity is perceived as equal to Forerunners in genetic evolution or care of the Mantle.

List of appearances

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