The operation to take the northern territories in the Terris Kingdom was always part of Longstride's overall plan for Melberg, but the thrust towards Atlas Prime was taking priority.
Then an unexpected turn of events meant that the scheduled push to Terracon suddenly received immediate attention. Since the defeat of Jeffries' armes in Narmala the Tau had unwittingly passed over an Eldar Webway portal. On the eve of the 23rd a cloaked figure arrived at the Tau forward HQ, bypassing the security around the complex.
The very next day Commander Longstride had agreed with Shadowstrike that the help of the Eldar would be accepted, even though the aliens were opposing their efforts in the Perseus Deeps. The Eldar had assured the Tau that they wanted nothing in return and that they only wished to see the Tau victorious on Melberg. They didn't say why.
On the 27th a hastily prepared attack began, with the Eldar attacking troop formations along the coast while Tau Hunter Cadres penetrated deep into Terris Kingdom. Initially the Imperium fell back rapidly, alarmed at the sudden intervention of the Eldar, but finally an immense battle coalesced out of the confusion around the city of Terracon. Three Space Marine Chapters also came to the aid of the Imperium as they desperately tried to halt the allied alien advance. The Dark Angels massed in strength, bringing more than half their chapter to bear in the north, while the Templars Redemptus and Emerald fists also bore the brunt of the fighting.
The massed forces of the Tau and Eldar converged all around the city and the largest set piece battle of the war so far unfolded around the city. Lord Commander Lenord threw his entire northern defence force into the battle, including the Librian 101st, 57th, 7th and 8th Grenadiers, as well as the Whettis and Terris Kingdom armies. Several thousand armoured vehicles took part in an apocalyptic and devastating tank battle in the plains outside the city.
The Tau were badly mauled in the battle as the Space Marines held the centre ground, but Lenord's armoured thrust, aimed at delivering a fatal blow to the Xenos alliance was suddenly and brutally blunted by massed and manouverable Eldar forces. Hundreds of Jetbikes and othe Eldar vehicles, supported by a mighty and rarely seen Revenant Titan tore into the Imperial armou, destroying more than two thirds of their number. At the same time two Super-Heavy tanks detonated in a shower of debris and fire.
The apocalyptic battle raged for hours with utter carnage on both sides until the Imperial forces were spent. The remaining Marines, mostly Dark Angels, fell back into Terracon, now reduced to rubble, as the Emerald Fists and Templars Redemptus forces tried to assess just how bad their losses had been. Of the Whettis and Terris Kingdom armies there was very little left. The Imperial Guard and PDF forces withdrew north and south, leaving Terracon surrounded as the Tau beseiged the city. Then, just as suddenly as they had arrived, the Eldar left, leaving their broken and twisted remains of their alien equipment littering the battlefield among the hundreds of burning Imperial tanks. The Carnage had also hurt the Tau badly, but now the entire northern half of Melberg was almost in their grasp.
While the Eldar assisted Shadowstrike in the north, Commander Longstride began the mch awaited assault towards Atlas Prime. He had no Eldar support but was confident he could overcome the forces of General Cunningham in the South. Cunningham however had also received support from the Emerald Fists and Dark Angels chapters and just as the Tau began their assault he launched his own counter-attack.
The eastern front had been so quiet for so long that the Tau hadn't realised just how strong the Imperial forces had become. The battle started with an accurate artillery barrage from the Basilisks of the redeployed 14th AVR regiment. In contrast to the woeful accuracy of the Librian artillery the Basilisks of the 14th AVR took a heavy toll on the Tau hammerheads and crisis suits as they waited in their attack positions.
The battle developed in a confusing and fragmented way along a broad front. Reeling the Tau tried to make good their shaky start to operation Xyston but were all too soon reacting to events rather than shaking them. Crucially the Tau forces deployed to meet the threat of the 7th Coronia Armoured Regiment which threatened to break through their left flank, rather than concentrating their forces towards the trategic objectives in the centre of the front which were weakly defended by the Imperium. By the time they did, belatedly, realise their mistake the Imperial high command had brought in fresh reserves, deployed by drop pod, in the centre of the front running across Coronia.
In the south the Tau faired badly as the 14th AVR massacred thousands of Tau fire warriors using massed Hellhound and Stormtrooper attacks. Already by the end of the first day Operation Xyston was a shambles and the Tau were in retreat. Though taking severe casualties the Imperial forces, aided by Astartes units, had done terrible damage to the armies of Longstride, devastating his Hunter Cadres and eliminating hundreds of Tau skimmer tanks.
Using members of the Officio Assassinorium to confuse and disorient the Tau. The Imperial commander had gained the strategic upper hand and soon began to exploit the weakness in the Tau centre. By dawn on 291007 the Imperial forces had cut a swathe into the Tau lines, broken through and reached the Gulf of Gwendelia. Once again the oilfields of the Coronia peninsular were in Imperial hands and Corunium was in danger of being cut off.
In the north and south the bloody fighting tailed off as both sides tried to consolidate their lines and forces. Though they had lost a huge swathe of territory in the north the Imperium had successfully turned the war in the south into a war of attrition, something the Tau could not afford to let happen. While Longstride tried to shore up his southern front Shadowstrike was already planning the next phase of the war. It would not be just yet however as the Tau no longer had sufficient forces to mount a significant offensive on any front, though fortunately for them, neither did the Imperium. The bloody war on Melberg would go on.