The Osir dynasty is a splinter dynasty that seceded from the Agdath Dynasty after the Osiris routing in the battle on planet Phaistos Osiris.
Overlord Xulis, also known as Xulis the Vengeful, was ordered to find and activate a long lost tomb on Phaistos to reinforce the ranks of the dynasty, which was plagued by the flayer virus and was in great need of fresh forces.
After arriving on Phaistos it was apparent that Necrons were not alone, and that the planet was not only already under control of the Imperium of Man, but also under siege from Chaos and splinter Hive Fleet.
Xulis decided that instead of foolishly attacking the foes on the planet with his small detachment, he would help the Salamanders chapter of the Adeptus Astartes with the defence of hive city Ravantium against the Tyranids. Salamanders accepted the proposal, where Xulis demanded that in exchange for the suprising reinforcements he’d be allowed to send his Necron scouts to search for a relic. In reality the scouts were sent to search for the hidden Tomb, and the Salamanders were most likely no fools to realise that, but the situation gave them no choice but to accept, for time was little.
The necron reinforcements hurried to the aid the Astartes in battle against the Tyranid horrors, and soon they started turning the tide of the battle.
But overlord Xulis had indeed greater plans and seeing the succes of his warriors he grew rather impatient. He was so bold and arrogant that he even sent his personal guard against the last hive tyrant, which resulted in the depletion of his elite units. Angered after realising what had happened and with the certain annihilation of the Tyranid attack he ordered his legions to turn their back on the Astartes and betray the temporary alliance. The predictable but quick betrayal caused the Necrons to wreck havoc inside the defenses, now turning part of fortifications of the hive city Ravantium against the Astartes themselves. Suprise attack seemed effective at first, but soon the great numbers of the Astartes furiously charged necron warriors inside tyranid devastated defences. It was then that Xulis got reports from the scouts that no tomb was present on the planet, and realised that no reinforcements would be coming.
He hurried to aid his warriors in battle but his powers couldn’t stop the Salamanders onslaught. As the overlord tried to ressurect his legions, charging Primaris captain struck his ressurection orb with his sword. With defeat at hand he faced the Primaris captain in melee combat. Surrounded by marines from all sides Xulis caught a mortal bolter round and fell.
After Astartes victory the space marines threw Xulises body into the wasteland. But the overlord was lucky that bolter round killed him, for his body wasn’t torn apart in melee and through pure anger and lust for vengeance Xulis reanimated his heavily damaged body. He raised his orb once more and gathered remains of his legions. Astartes got caught in combat once more with other xenos races, which gave the Necrons time to retreat from the planet.
The Necron overlord knew who was responsible for the slaughter, the lying Agdath Dynasty. The fact that no Tomb world was found and no reinforcements were sent angered Xulis so much that he had better plans for his legions. With the Agdath dynasty weakened by the plague and with his feeling of betrayal he seceded from the Agdath dynasty and proclaimed the Osir dynasty, in the name of the planet where he was betrayed.