After The Didact (whether this was Ur-Didact, or Bornstellar-Didact is unclear, just like we don't know which Didact is in Halo 4, either, just that he is not the same Didact the activated the Halo Array 100 000 years ago) defeats the Ancient Humans lead by Forthencho the Lord of Admirals on Charum Hakkor (the capital world of the Human-San'Shyuum alliance), the remaining humans, as well as the Lord of Admirals, are held alive in some structure/tomb on The Capital (the huge megastructure that dwarfed the Halos that served as the political centre of the Forerunner Ecumene). The Forerunners decide to archive all the remaining Ancient Humans' minds and memories using the Composer, which destroys their bodies. The Librarian and other Lifeworkers eventually imprint this "saved-state conciousness" unto various primitive humans in an attempt to hopefully reveal how the humans supposedly cured the Flood (we later learn that Ancient Humans did not actually cure the Flood, which really pisses ONE of the Didacts off at one point). Forthencho, the Lord of Admirals, is imprinted onto Chakas thousands of years later (and he actually has conversations with him from time to time). Chakas then eventually get's Composed, turning him into 343 Guilty Spark. Also, the Lord of Admirals main political opposition, and the "leader" of the Ancient Humans' science and technology, Yprin Yprikushma, is imprinted on Riser, Chakas' friend. You see this briefly alluded to in the terminals in Halo 4, too, when the Didact goes to the Capital and locates the Lord of Admirals, telling him of the Forerunners' plan.
There is a section near the end of Primordium with the Lord of Admirals, within Guilty Spark, who is very badly damaged after the events of Halo 3, but recovered by a UNSC vessel. The UNSC interrogate 343 GS, but the Monitor discovers that the Librarian is still alive, takes over the ship, making all the humans aboard sleep, and takes off in some direction in space (likely toward the Greater Ark, which is Installation 00 Ark's "big brother", responsible for the creation of the original 12 Halo rings, which were all 30 000 km in diametre, rather than 10 000 km the "new" Halos built by the lesser Ark that we see get destroyed at the end of Halo 3.) At one point the Lord of Admirals speaks with the UNSC, before 343 takes over.
