I was re-reading the Halo 3 terminals and I feel like it contradicts the events in Halo 4. In the terminals it makes it seem like the Didact was on the Ark, firing the Halos while the Librarian stranded herself on Earth. However, in Halo 4 it seemed like the Didact was imprisoned on Requiem before the Halo's were fired.
I didn't read the newer books, but am aware that there are Ur-Didact and Bornstellar(?) Didact. Could this confusion have something to do with the difference between the two?
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At the moment it's unconfirmed who's who, but in Cryptum I remember reading about how the sudden "transformation" caused Bornstellar to look uglier than usual. That's what I based myself on to think that the Didact from Halo 4 was the Bornstellar Didact.