The Imperium has tried to hold the spaceports against the forces of the Tidebringer Enclave and the Darkstar Armada. In particular, the Khorne warlord was enraged to find out Ahriman had left the system, stranding the Khorne host to fight their enemies alone. The forces of Tzeench had, after all, accomplished what they'd come to do which was to awaken the Necrons and throw the region into disarray.
In the end, the Imperium have been largely pushed off the spaceports and they are desperately fighting for their lives behind the growing enemy lines.
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