SUMMARY
After losing political power to the Master Builder and the entire Builder rate following the end of the Human-Forerunner Wars, the Didact placed himself in exile within a cryptum on Nomdagro, which was later moved to Erde-Tyrene. While the Didact held significant power within the Forerunner government after his victory against ancient humanity, and used that power to build many shield worlds, his defense against the Flood, the Master Builder eventually won the political fight, convincing those in power that the Halo Array was a better and more effective plan. Having lost all political power and facing public shaming, the Didact chose to hide away in a cryptum until he was needed again.
RELATED MEDIA
These events are referenced in the novel Halo: Silentium, which was the subject of a Halo Book Club on January 31st, 2018.