Night Falls Upon Crastus

The campaign known as the Night Falls Upon Crastus was a dark period in the history of the Obsidian Blades Space Marine Chapter, which saw their home world of Crastus being subjected to a large invasion by Aeldari corsairs. But soon a more dire threat revealed itself, in the form of the undying Necrons, who had occupied the planet millennia earlier, and was in fact, a Necron Tomb World. During the fighting between the Space Marine and the Aeldari, the Necrons were awoken from their slumber and proceeded to attack both sides. Overwhelmed by the Legions of undying metallic warriors, the Obsidian Blades' fortress-monastery, along with most of their genetic heritage and a good portion of their amroury, were destroyed. This put the Chapter in dire jeopardy, and forced them to reassess and take stock in their tactics and standard operating procedure, forcing them to adapt a new way of fighting which would become their hallmark.

History
For the second time, the existence of the Obsidian Blades was put in jeopardy, this time, by the duel machinations of the Aeldari and the Necrons. Unbeknownst to the Obsidian Blades or their Death World inhabitants, countless millennia before, the world had been the used to hold the ancient, slumbering Necrons, and they had started to awaken.

Drawn to the Necron threat by their own inscrutable means, Aeldari Corsairs descended upon the planet, slaughtering the Death world inhabitants and setting the planets jungles ablaze, its deserts awash, and its mountains to dust. Their awakening accelerated by the Eldars presence and their global devastation, the Necrons marched forth from their catacombs and made war against the Aeldari across the surface of Crastus, with the Obsidian Blades caught in the crossfire.

With their fleets and companies away and the skeleton crew of an already sparse chapter left behind, the remaining Obsidian Blades were unable to utilise their infiltration speciality, stuck as they were defending their fortress monastery and their invaluable gene-seed. Fortifying as best they could, they shored up their monastery and made best they could a last stand, all the while calling for reinforcement. None would hear their plea.

By the time reinforcement from the Obsidian Blades fleet made Warp transition, the war had long since ended. The Eldar, in an attempt to prevent a full scale Necron awakening, initiated a devastating series of xenos bombs at fault lines around the planet, the resulting planetary destruction was incomprehensible. Whether the Necrons survived the destruction of the planet, they did not know, there was no living sign of the mechanized aliens. So too had the Eldar returned to the Webway, having executed whatever indecipherable plan they had set out to achieve. Of the Fortress-Monastery, little remained, some sealed vaults survived the destruction, containing the holiest of the chapters relics, and samples of precious, precious gene-seed, but over 90% of the Chapter's history and genetic lifeblood was gone. Once again, they faced extinction.