The Battle of the XoStaurus system, as it's officially known in Imperial records, was a battle between the Necrons of the Atekph Dynasty, the Imperium of Man and a detachment of the Knights of Redemption, and against a yet unknown Chaos warband. It's commonly known as the Warp storm of XoStaurus.
The Imperium of Man had long occupied the planet, unaware to the presence of the Necron dynasty, the Atekph Dynasty, below the surface. The Imperium had managed to analyze many weapons that were left behind on the planet by the Necrons as having been built by Necrons, although Imperial records had no logs or information on a Necron Dynasty being located here, causing the Imperium to assume that the Atekph Dynasty had perished during the War in Heaven.
However, a majority of the Dynasty was burried below the surface, and only Nemesiar the Restless and his Crypteks were awake during the Imperial occupation. However, the sudden appearance of a Warp Storm woke up the Necrons on XoStaurus Prime. Nemesiar had started planning to reclaim XoStaurus as theirs, and now he had the perfect opportunity. A Chaos Warband came with the Warp storm to invade the planet and take it for the Forces of Chaos, as the planet was a fortress world.
The battle - in space
The Warp Storm arrived, and brought with it, a Chaos Warband, set on the conquering of XoStaurus. The emergence of a Chaos warfleet immediately triggered a space battle between the Imperium and Chaos. However, while the space battle largely occupied the Chaos force, several hundreds of Chaos warriors made it through and managed to land on the planet. They were dislocated and misplaced, causing drastic casualties already, in addition to being bombed by the Gauss Monolith, which further demoralized Chaos.
The Necrons on the ground were in the meantime awaiting the perfect moment to strike, as Nemesiar had decided to bring up his Necron warriors behind the Space Marine lines. Nemesiar wanted to take out the Necron weapon used by the Space Marines first, either by capturing it back or destroying it.
When the Chaos forces finally regrouped and launched a more succesful assault against the Space Marines, Nemesiar decided that it was time. Nemesiar and his Crypteks crawled up to the fortress, preparing to retake the Gauss Monolith themselves, while the Necron Warriors would deal with the other Imperial forces. Drent Knight, the leader of the detachment of the Knights of Redemption, was shocked by the sudden appearance of Necrons, but did not hesitate to fight them.
With the appearance of Necron ground forces, several Scythe-class Harvest Ships had also appeared to face the Chaos and Imperial fleets. While the Imperial and Chaos fleets outsized the Necron fleet, the Chaos and Imperial fleets mutually destroyed eachother in such a manner that it didn't matter, as by the end of the conflict, the majority of the Necron fleet was still alive, and the other fleets were almost completely destroyed.
Drent entered hand combat against the Necron Lord himself, but was quickly defeated against the far superior Necron Lord. Nemesiar and his Crypteks had quickly reclaimed the Gauss Monolith, now using it on scattered Imperial forces and regrouping Chaos forces.
Drent Knight had realized that defending the planet was futile, and ordered contacted Chapter Master Aaron Rahn. Before the Chapter Master was able to say anything though, the Imperial commander had commanded a full retreat. Drent Knight, which was very reluctant about retreating, considered shooting the Imperial commander, but eventually agreed with him. Like the Imperial commander, Drent and his forces entered the transports sent by Battlefleet Sabbat, turning the battle into a two-way battle between the Necrons and Chaos.
The Chaos champion, whose only goal was to destroy the Space Marine forces, also ordered a retreat, although his retreat wouldn't be nearly as smooth as his counterpart's retreat. The champion was forced to leave many Chaos warriors behind in order to save the majority of his warband, and eventually, the Champion escaped the planet.
The battle - XoStaurus system
While most of the fighting took place on XoStaurus Prime, the three fringeworlds in the XoStaurus system had sent reinforments and were the source of the Necron space ships that arrived at the space battle. Small space battles took place over the other three planets, although all of which were Necron victories.
Following the battles of XoStaurus and the XoStaurus system, XoStaurus and the other three planets in the system had become Necron territory once again, with the remaining Necrons awaking from their deep slumber soon after, and the Imperium realizing that they woke up another Tomb World.
The Imperium was forced to give up occupation of the planet, and only a small portion of the PDF was able to retreat. The 4th company of Drent Knight was able to pull out with the majority of its forces in comparison, which retreated back to Isolum.
The Chaos Warband remained unidentified until after the battle, although it had taken considerable casualties during the battle, including its heaviest of troops. The Warband managed to retreat from XoStaurus with the majority of its remaining troops.