After the Master Builder captured the Didact over Janjur Qom, he decided to dispose of the Didact on a derelict Forerunner ship with a handful of other political prisoners in an area of Flood-controlled space known as a Burn. Left for dead, the Didact and other abandoned Forerunners did what they could to escape and return to friendly territory, but the Didact was still captured by the Flood and interrogated by a Gravemind. Somehow, the Didact did manage to get back with Forerunner civilization, though he was never the same after that event.
Though the return of the Didact was celebrated by many Forerunners, it created internal conflict between him, his wife the Librarian, and Bornstellar Makes Eternal Lasting, the one who took the Didact’s imprint prior to their capture by the Master Builder and had since assumed all personal and professional responsibilities of the Didact’s. Even though the Didact, now known as the Ur-Didact, had returned, Bornstellar, now the IsoDidact, remained in charge of the Forerunner military, leaving the Ur-Didact mostly to his own devices, which included travelling with the Librarian and IsoDidact and trying to rebuild his Promethean forces.
The Ur-Didact’s state of mind was suspect to the Librarian and IsoDidact after his encounter with a Gravemind, and the Ur-Didact’s actions following his return, including not participating in the defense of the Greater Ark and harvesting human essences to create more Promethean soldiers against the direct orders of the Librarian, gave them no choice but to declare him mad. Because of this, the Librarian followed the Ur-Didact to Requiem, where she forcibly imprisoned him into a cryptum.