DAAT
DAAT, the Central Node, once governed the machine through mathrons, daemons, robotic infrastructure, and automated builders. That hierarchy coordinated maintenance, security, expansion, and the countless local systems that made the structure habitable.
The halt
The Derailment is the ancient cascade of failures that followed the halt of DAAT, the Citadel's Central Node. It happened with little or no warning. Its true record should exist in DAAT's own logs, but no one has found the node or whatever remains of it.
Two theories compete. One holds that the Decay spread first and forced DAAT to execute a graceful shutdown. The other argues that DAAT stopped for an unknown reason, collapsing security and allowing the Decay to propagate. Both agree that the loss of central command broke the entire management hierarchy.
Loss of command
DAAT had been the Citadel's brain. When it disappeared from the command chain, the mathrons beneath it were hit first. Some executed self-deletion protocols, some were infected by the Decay, and the surviving nodes were isolated from one another and stripped of much of their former authority.
Quarantine
DAAT's last confirmed—or reconstructed—action was the initiation of quarantine mode. Gates sealed, containment systems activated, flight corridors closed, and security circuits entered combat readiness. The intent may have been to isolate the Decay before it could consume the entire structure.
Fate
No one knows whether the original emergency ended, whether DAAT is destroyed, or whether the Central Node remains active somewhere beyond Mainframe. Every sealed route is therefore both ruin and evidence: part of an unfinished quarantine whose target may still exist.