Between the first Forerunner book, Halo: Cryptum, and the first chapter of the Reclaimer Saga, Halo 4, the character known as the Ur-Didact underwent a horrifying transition. He started out as a noble and reasonable warrior, and ended as a genocical madman with a clouded mindset on conservative Forerunner values.
The legend goes like this. At the end of Halo: Cryptum, the Ur-Didact was captured and presumably executed by the Master Builder, Faber Of Will And Might. In truth, the Didact had simply been exiled…stranded and left to die with three other Forerunners, all in a star system overrun by Flood. Over the course of their exile, the Didact and his newfound companions would be captured by Flood. This was the Gravemind’s first words to the Didact once the Forerunner ship they were in was surrounded by Flood:
Didact, do you have a moment? Just a moment? That’s all it will take.
We don’t know what happened after that line, only that when the Didact was released by the Gravemind (rather than being infected like what literally happened to every other being), he had lost his sanity. Like, he’d been literally mind-raped (and not the Gravemind tentacle hentai way). Anyway, upon returning to the surviving Forerunners, the now-insane Didact proceeded to act accordingly. Now at loss of judgement, at the time when stability among the Forerunners was most important, the Didact used the Composer to wipe out the Librarian’s entire population of humans she had kept on Omega Halo. In revenge, and to safeguard what was left of humanity, she imprisoned him in Requiem, locked him in a Cryptum, and hoped that he would meditate with the Domain to regain his sanity and eventually help humans reclaim the Mantle countless generations later.
That plan also went to shit. Because just before the IsoDidact fired the Halo Array, the Gravemind informed the Librarian that the Domain was a Precursor artifact, and would thus be destroyed within the next few minutes. In other words, the Librarian (all praise her genius) condemned the Didact to 100,000 years of living alone in a cramped metal ball without wifi.
Anyway, onto the theory. The question of concern is what, just what, happened between the words “Didact, do you have a moment? Just a moment? That’s all it will take” and him returning to the Forerunners. What did the Gravemind do to him?
The answer could lie within shadowy methods presumably used by the Soviet Union during the Cold War. The Gravemind, while wearing badass Russian snow hat and speaking in a Morgan Freeman-tier Russian accent, brainwashed the Didact to turn against his allies upon hearing a trigger phrase (Family Guy tells us that the trigger phrase is “Gosh, that Italian family at the next table sure is quiet!”). When the Didact, now fully brainwashed by Soviet manipulation, returned to the Forerunners, we can imagine that some asshat probably muttered “Gosh, that Italian family at the next table sure is quiet!” which in turn made the Didact go full slave trade against humans inhabiting Omega Halo.
We can likewise assume that the Didact keeps on hearing the fucking numbers.