From Halopedia, the Halo wiki
Shadow-of-Sundered-Star, better known as the Ur-Didact, is a Promethean who once commanded the entire Forerunner military. However, an encounter with the Gravemind left his mind and psyche permanently altered. One hundred thousand years after being imprisoned by the Librarian on Requiem, the Didact was freed by John-117. The Forerunner retrieved a powerful device known as the Composer, but was knocked into a slipspace rift while trying to compose the population of Earth. This rift took him to Gamma Halo, the location of the Composer's Abyss. John-117 and the rest of Blue Team confronted the Promethean and digitized him with the combined power of five Composers. Now, the Didact wanders the digital wastes of the Domain.
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