Yeah, the original Didact and his clone. Original (or Ur-) was previously executed in the books, but H4 has retconned it and had him imprisoned on Requiem instead.
Bornstellar, his clone, took his place in Forerunner society and was much less extreme, especially with regards to Humanity. He was the one who activated the Halo Array.
Okay, I was unaware of the retcon and I assumed it was Bornstellar the whole time, and The Librarian remotely activated via The Ark after she imprisoned him. My retcon makes much more sense, Where the hell is Bornstellar at when all the composing of humans is going on?
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That, plus, Silentium is going to have the Ur-Didact as a main character, with no information whatsoever about Bornstellar. Which leads me to believe the Librarian betraying Bornstellar took place between Primordium and Silentium; and then the Ur-Didact comes back in SIlentium, and is the one to ultimately fire the Halo arrays.
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