Pirates of the Oblivion Abyss.

In the year 222. M40, the Ordo Malleus condemned Oblivion Primaris under the leadership of Lord Inquisitor Irvine Fathul. The Skyworld had become overrun with savage Orks and maddening entities of the warp. With the lack of Cyclonic Torpedos or Virus Bombs available, the world was ordered to bombarded for six months by numerous Regimental fleets, the Battlebarges of the Astartes under the banner of the Adamantine Guard. After they were certain that no life, organic or Immaterial, could have survived, they left the rock ball behind and headed to the frontlines of the carnage of Waaagh! Execooshuner (On a side note, Boss Execooshuner gained his name by having an Executioner cannon in place of an arm).

The Eldar, however, knew the secret of this world. Beneath the lung burning atmosphere and the acidic, light blocking clouds slept something that could not be allowed to awaken. The human bombardment had awoken the dreaded Necrons from within their catacombs. The Humans had faced resistance from Raiding Eldar at the time, but thought little of it and fought back with extreme prejudice, overwhelming them.

Only a few Imperial ships were left to keep watch over the system, leaving it rather vulnerable.

Bright jade fires opened within the shadow of Oblivion Primaris's second moon, from it a bladed carrion fleet lurked from the nightmarish depths of the Dark City into reality. Upon the Flagship's Bridge sat Dread Archon Vorshulth, accompanied by the enigmatic Harlequins. He had not even considered going against there wishes, as he had learnt in his years as a captain that there was more horror in the real world than he could ever learn about in Commorragh.

The Harlequins had insisted that he bring an unusual weapon, the likes of which he had never seen before. It had been crafted by the Twisted Flesh Coven and commissioned by Grand Archon Dernia Cavash himself. If the Grand Archon had been in on this operation, then he would have been foolish not to take part.

The weapon had been far more powerful than he had expected. 'Sanctity' as it had been named, slowly drifted towards the blackened world, and impacted with unimpressive results. The Dread Archon dug his nails into his throne's arm in anger at the resources he wasted for this mission, all in vain.

Then it detonated.

The small spark upon the surface began to consume the world, a great vortex of startling blues and reds annihilating the planet completely. As the vortex grew tore open everything it had come into contact with. Moons were torn apart into asteroids that orbited the maw.
The fleet, however, was far too close.

Many of the ships were taken in by the implosion, and others became disfigured by the inward pull of the anomaly. Realising this error, Dread Archon Vorhulth ordered entry back into the Dark City. The jade fire Portal opened once more, and even with the flagship moving at full speed, it was locked half way in an halfway out of the Portal. Skyscrapers reached out of the fire, their tips shattered by the vortex. Millions fell into to the Warp Storm that had been opened, their souls forever damned.
Eventually, however, after a hard struggle, the ship broke free and found its way to safety, shutting the Webway Portal behind it.

The carnage caused throughout the Port-realm he had launched from severely outweighed his victory and he was soon chased into exile.

It took weeks to repair his flagship, and with a diminished crew it proved difficult to fly. Desperate, he turned a beady eye to the Imperial sentinel force. It took a lot of murders, in both his own ranks and the Imperial's, but he had convinced them that their Emperor had abandoned them, at that by joining his fleet they could seek vengeance against those who had abandoned them; but most importantly, survive.

Over many centuries, he assembled a crew of Imperial Guardsmen, Tau, Kroot, Eldar Corsairs, four armed K'tari and four Traitor Marines of the Night Lords Legion.

Soon, he returned to the area designated the Oblivion Abyss and oversaw the construction of a Genesis Spire by the Eldar Corsairs he had hired. From his base he struck out against Imperial supply lines and Ork Waaghs, bringing them to the asteroid belt that surrounded the Warp Storm. The Storm itself had become much weaker by the end of the Spire's construction, causing it to hold the field in a spiral form, instead of consuming it completely.

Bringing the Orks to the field was purely tactical, as it defended them from the prying gaze of the Imperium.
From their shielded base, they reigned terror throughout the Oblivion sub-sector.


Edit - I apologise for the poor writing quality and description, I didn't have long to write it.