The pace of the game of chess between the forces of Chaos and the Imperium quickened to all out war in 11.009M42. Imperial strategists knew that the source of Chaos power in the Aleph sector lay in the dark systems within the lawless Perseus Deeps, an area of old dark stars which while falling within the Imperium's claimed territory, was largely out of control. At its centre, Bastien, Mordecai, Calliden and Kendrenec dominated the Deeps, Chaos systems all within easy reach of a known warp lane leading directly to the eye of terror.
With the Rim worlds under Chaos control and the war on Alphe in the balance, Imperium high command directed Battlegroup apocalypse, supported by several of Jellicoe's cruisers to head for the system of Sullidan, a small base in the Deeps nomincally controlled by the Imperium, though there hadn;'t been any official contact with the population of some 10,000 in many years. The force would include a large army of around half a million men from Corellia, Libria and Porphyria, as well as a whole regiment of super heavy vehicles. This would be the first vanguard force intended to secure the planet, before building up an even greater force to support an invasion of Calliden. Once Calliden had fallen the crusade would move on to the other Chaos worlds, finally leading to an all out assault on Kendrenec. This great move would finally crush the rising power of Chaos in the sector. The Night Lords however declined the invitation, sensing a trap.
As Battlegroup apocalypse got underway, they could not have known that their plans were known to the enemy. The Claws of Lorek, having recently conquered the system of Yaasha, gathered around them as many allies as they could, even luring Da Verminator's Orks into the fight with the promise of loot and a good scrap. The Claws convinced Shaidar Haran and Admiral Thok to convey them to the Sullidan system, and even the Catechism sent a force, albeit one of expendable traitor Guard from Cerberex, to ensure that any Chaos victory would not be achieved by the Claws of Lorek alone. Moonface would have some of the glory. Another force, now known as "The Anticross", also joined the force of arms, ensuring that when the Imperium arrived they would face overwhelming force.
On 1211.009M42 Battlegroup Apocalypse had arrived in orbit around the planet of Sullidan and had disgorged their massive force on the planet, finding unexpected welcome from the citizens who were apparently glad to see such a force. Had they known what was coming, they may not have been so welcoming. Days later the Imperial battlegroup received reports of a massive Chaos fleet arriving in a neighbouring system. Believing Sullidan to be secure the battlefleet moved towards the system jump point in order to give battle, only to find the Chaos battlefleet apparently leading them in a strange game of chase around several star systems for nearly two weeks. They soon found out why.
With the Imperial warships drawn off, the forces of Chaos easily brushed aside the picket ships left to defend Sullidan when they arrived a few days later. After a brief skirmish in space, the Claws of Lorek led the landings on the planet itself, somewhat surprised by the strength of the Imperial force already landed. An apocalyptic showdown was inevitable and occurred on 1411.009M42. Both sides understood what was at stake. The victor on Sullidan would hold the initiative in the Perseus Deeps, and the warmasters of Chaos had their own invasion plans to consider.
The battle itself was a violent affair as the Imperium drove towards the Chaos lines with a simple yet brutal plan - to pour as much firepower into the Chaos forces as possible and to annihilate them utterly. On the left flank the Corellian Storm Guard moved forward through wooded terrain, while in the centre the Librians gave fire support from the edge of Sullidan's one city, blasted to rubble by an intense preliminary bombardment. On the right, the Porphyrian Guard prepared to hold the line against a horde of orks.
The Chaos plan was more subtle. Thanks to poor shooting, especially from the super heavy tanks, the rhinos of the Anticross weathered the storm of Imperial firepower, and the Catechism forces were able to move their armoured units forward, duelling with the Librian tanks. On the Imperial right flank, despite suffering horrendous casualties, the Orks closed with the Porphyrians. On the right, some dithering by Porphyrian and Corellian units meant that the traitor forces were able to hold them at bay with barely more than a few squads of grots.
As the day wore on the Imperial commander, General Veers, felt victory was inevitable. A formation of Hellhaounds on the right immolated most of the Orks and finally the super heavies were advancing on the enemy, with the forces of the Anticross suffering at last. Meanwhile the Librians were locked in a titanic struggle with the Catechism Guard units, concentrating their fire on the traitors, though the support from their baneblade was largely inaccurate. Then disaster struck. On the Imperial left a flank attack by airborne Catechism units, something Veers had not anticipated, was supported by Ork deathkoptas caused mayhem amid the Corellian Storm guard, halting their advance. On the left the Claws of Lorek, shielded by the Orks, barelled into the Porphyrians and almost broke the Imperial lines.
Then, as the Imperium was reeling, the killing blow came. The Claws of Lorek mounted a massed teleport attack by Obliterators, and though many were lost in the assault, the carnage inflicted was enough to halt the Imperial advance cold. Soon the Catechism Guard units had regained the initiative in the centre and poured into the Librian lines. Mega armoured Nobz joined in, destroying a super heavy tank, and two more exploded in catastrophic magazine detonations, causing heavy casualties on both sides. One of these had been the intended prize of Da Verminator, the Catechism artillery making sure the Orks would not get their hands on such a machine.
After four hours of chaotic fighting General Veers realised he had lost control of the situation. The Catechism had taken the centre of the city, badly mauling the Librian regiments. On the right the Claws of Lorek had completed their advance and on the left the burning remains of a Shadowsword bore testimony to the savage fighting. Here too the forces of Chaos were in control and reinforcing their positions. The Anticross forces too had recovered from their losses and the Imperial commanders knew the battle had been lost. General Veers retreated his forces into the wastelands of the planet, abandonning the system to Chaos. Four days later the battered remnants of Battlegroup Apocalypse were evacuated from the surface of Sullidan.
The outcome of the battle was felt immediately. The invasion of Calliden was postponed, and the Imperium began reinforcing systems close to the Perseus Deeps, including Farport in the Vastrid subsector. Later a psychic scream in the warp bore witness to the creation of a daemonworld, named "Carnage", where Yaasha once existed. Concern grew that the power of Chaos was rising and some in the Imperium began considering an alliance with the Tau, though not openly. The battle of Sullidan had been the opening shot in the great war against Chaos in the Aleph sector, as the power of the great enemy grew to heights not seen in a millenia.