From Halopedia, the Halo wiki
Keth Sidon was a world in the Forerunner ecumene. The planet was home to a former member of the long-defunct Theoretical rate who had an extensive collection of ancient literature, including a preserved physical copy of the long-suppressed works of a Theoretical known as Boundless. A century before the Didact entered his first exile in a Cryptum, the Librarian discovered and studied this copy in her search for the truth on why Boundless had been forced to cease her studies.
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