From Halopedia, the Halo wiki
The Builder was one of the few Forerunners who survived the activation of the Halo Array. Soon embarking on a search for Bastion, he discovered that Bastion had been moved from its original location, a feat he believed would require "an impossible act of reconciliation" to perform. The Builder was desperate enough that he sought out a surviving durance and enlisted an ancilla to help him find a route to the Domain, where he hoped he could make it to Bastion. His search took him to Sanghelios at one point, where he believed he had found a route to Bastion. Instead, he arrived at Genesis, where he found that the Domain had been severely damaged by the Array, and was trapped there, degrading over time.
Surviving in Genesis' systems, the Builder witnessed the beginning of the alliance between Cortana and the Warden Eternal, and reacted with alarm as they took control of Guardians across the galaxy to enforce the Mantle of Responsibility. He noted, however, that the Domain had repaired itself enough to cure Cortana of her rampancy, and hoped to finally access it. Eventually, Cortana noticed his presence in her network and began to hunt him down, though the Builder left audio logs scattered across Genesis to warn the human forces who opposed her, noting that Cortana had not yet taken control of the installation's Constructors, and that the installation's monitor, Exuberant Witness, could use them to take back control.
As Cortana began to deploy her massed Guardian forces across the galaxy, and the Warden was distracted fighting both Blue Team and Fireteam Osiris, the Builder was able to finally access the Domain, restoring him and finally allowing him to locate Bastion. His fate afterwards remains unknown.
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