XoStaurus, also often called XoStaurus Prime, was the primary planet of the XoStaurus system. It was a crownworld of the Atekph Dynasty and was orbited by three fringeworlds, also ruled by the Atekph Dynasty.
XoStaurus was a planet located within the Necrontyr empire and was the crownworld of the Atekph Dynasty. While formally not being a part of any realm, the rest of the system was also considered part of Atekph Dynasty territory. When the War in Heaven fired, XoStaurus was of paramount importance to the Atekph Dynasty, and was transformed into one of the best defended planets in the Necrontyr Empire.
After the War in Heaven, XoStaurus became a forgotten world, and it took many thousands of years for the Imperium of Man to find it again. The planet was deserted, according to the Imperium of Man, unaware of the Necron threat that lay below surface. XoStaurus became a military stronghold for the Imperium of Man, using all the foreign weaponry that the Necrons had installed many thousands of years ago to their potential.
A detachment of the Knights of Redemption was stationed on XoStaurus during the occupation of the planet. The Necrons had been long asleep, with only Phaeron and his Crypteks being awake, and fully aware of Imperial occupation. The Necrons had awaited the most opportune time to strike, as even with their technology, it would still be a difficult battle to fight, as the Imperium had occupied several of the Necron weapons as well. The most notable of which was a Gauss Monolith, a variant upon the normal Monolith, armed with a large superweapon that could obliterate the heaviest of Imperial tanks.
The opportune time arrived, when the sudden appearance of a Warp Storm brought the Forces of Chaos upon the planet, triggering a battle between the Imperium and Chaos, soon to be joined by the Necrons. Eventually, it was a Necron victory, and XoStaurus was fully awakened by the end of the battle. The Necrons reclaimed almost all of their weaponry, with the other weapons being destroyed by the Imperium beforehand.