From Halopedia, the Halo wiki
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Bastion was a location sought by a surviving Builder who left audio logs scattered across Genesis. While Bastion's exact nature was not detailed, based on its description by the Builder it was a planet-sized Forerunner installation capable of being moved through slipspace, even after slipspace had been destabilized by the deployment of the Halo Array, though the Builder believed such an act would require a nearly impossible act of causal reconciliation. It was important enough that the Builder was willing to lose corporeal existence and resort to existing within a durance and connecting to the Domain to reach it, but was unable to find it due to it being moved, and instead became trapped on Genesis. In late 2558 the Builder accessed the Domain while Cortana and the Warden Eternal were distracted, and managed to finally confirm Bastion still existed.
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