The Imperium had bases on the inner planet, but the main archeotech work was taking place on Ceti, orbiting the Ceti sun more distantly than the necron tombworld. Although cold, Ceti was habitable, and the Adeptus Mechanicus had made significant discoveries by 03.021M42. Protected by the armies of the Forge world of "Iron Maiden", the Imperium had no concerns about the status of the operation. Protected by an army and defence lasers the commanders of the expedition felt secure. They were mistaken.
As the Mechanicum dug deeper, they encountered vast chambers under the ground, and a mysterious giant portal that appeared to be inoperative. Suddenly on 0403.021M42 the portal sprung into life, triggered by the clumsy machinations of the humans digging around the protected ruins of the Necron dynasties. A vast necron army swarmed out of the gate, crushing the expedition from within before moving onto the surface and facing the army of the Adeptus Mechanicus.
Moving with unnatural speed the necrons swarmed over the Imperial positions, gradually pushing the Adeptus Mechanicus forces back. The imperial forces called for aid, but there was no force within range to rescue them from their predicament. In just a few days the remaining Mechanicum forces were scattered into the wildernesses of Ceti, while the necrons established their new dominion over the outer worlds. Barely registered in the sector capital at Vastrid, the new necron threat was mostly ignored. Whether this would be to the Imperium's detriment, only time would tell.