WesleyShark

Yep, thanks to the Librarian betraying him and locking him up in a Cryptum. When the Halos were fired, the Domain was extinguished, and Ur-Didact was left to meander in his own mental rage and torment for a very, very long time. Bornstellar/IsoDidact, if still alive, is out beyond the galaxy somewhere.

Yep. There's an extra epilogue for Silentium, and in it

There are two possibilities of Didact returning in Halo 5 (or in subsequent titles): Ur-Didact: He wasn't killed by MC at the end of Halo 4- he even speaks in the epilogue, most likely to his army of Prometheans at an undisclosed location and time. He fell backwards into the slipspace portal, the same portal that was being used as a conduit for the data stream of composed humans from New Phoenix. He ended up going to wherever that stream was being fed to, more than likely. Where might be that "wherever" be? - The Lesser Ark - Halo Installation 03, or perhaps one of the other "undiscovered" ones. - Another shield world (we know there were many constructed. We've only known about 3 different ones so far, I believe?) - Somewhere new and unknown as of yet. Either way, he most likely ended up somewhere quiet far away from the solar system, so he'll need to regroup and find new means of travel. IsoDidact: He vowed to never return and meddle with the affairs of this universe, but I definitely could see him returning somehow/for some reason. Perhaps he finds greater truth (or danger) out beyond our spiral arms, and has no choice but to come back. Or perhaps he finds Precursors or like-Precursors out there. Who knows. That's so very open-ended at this point, and he'll either come back, or it'll be forever a "loose end" of sorts. Nothing wrong with a bit of mystery. :]