The Didact was a full-rate Promethean of the Warrior-Servant class in the Forerunner class system.
He earned his fame in the Human-Forerunner War. After the War, the Forerunners began to research ways of stopping the flood. This lead to the invention of the Halos.
The Didact was opposed to their use and was forced into exile by the Master Builder to a Cryptum. His Cryptum was placed on Earth by his wife, the Librarian. It is opened 1000 years later by a young Forerunner of the Builder class named Bornstellar Makes Eternal Lasting .
The Didact passes on his memories to Bornstellar and is later killed by the Master Builder on the San'Shyuum home world. Because of the transfer of memories, Bornstellar becomes the Didact. This revelation is made to him by the Librarian when he meets her on the Ark. From this point, the Didact begins to lead the Forerunner's fleet against the flood.
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".
The 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, Requiem , or one of the Halo Rings.
While the Server's first episode seems to state that the Didact perished, Terminal 7 seems to contradict this, as the 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.
