The planetary system Omicron Persei lies around eleven parsecs outside of acknowledged Tau territory and lies in a region of space known as the Boolean Nexus. Large and volatile nebulae make passage through this region of space notoriously difficult with only a thin corridor existing allowing navigation through the Nexus.
Sitting at one end of the Nexus corridor lies the Omicron Persei system, a gateway to the Eastern Spiral Arm and the distant worlds it contains. This has led to many a conflict raging through the system due to its strategically important location. Warp travel is possible but is often lengthened due to the proximity of the Nexus, thus making the system a prize for many civilisations.
The most highly sought after world in the system is Omicron Persei 8, a verdant world with a breathable atmosphere, lush rolling plains but devoid of an indigenous humanoid population. Omicron Persei 8 has changed hands many times over the course of history however it is currently contested by two forces, the Tau and the Imperium of Man. The planet is divided into two land masses on the eastern and western hemispheres, conveniently one is held by the Tau and the other by the Space Marines sent in awaiting support from other Imperial forces.
Forces are at a stalemate at the moment as neither side seems to be able to breakthrough to establish a beach head on the opposite continent. It was highly unusual then when a delegation of Craftworld Eldar showed up and tried to broker a deal with both side, sighting another threat far greater than the enmity between the Imperium and their Xenos enemy. Why exactly the Eldar had turned up was not evident to either of the two warring parties but the Marines were not prepared to yield and neither were the Tau.
While the Eldar were not drawn into the conflict at this point the fighting between the two entrenched forces continued, the Space Marines waiting for reinforcements to arrive in order to crush the Tau resistance and claim the planet for the Emperor.
Within days of the warning from the Eldar however another set of signals appeared in the system. Rather than the expected Imperial reinforcements a contingent of Chaos vessels were spotted. Neither the Tau nor the Space Marines were willing to lose ground to another enemy and for a while hostilities between the two ceased as running orbital battles were fought to see off the Chaos incursion. This was deemed to be the fulfilment of the warning from the Eldar.
Despite their best efforts the uneasy alliance of the Tau and Space Marines were unable to stop Chaos getting a foot hold on the Tau held continent, the Eldar were also suspiciously absent from the fighting. Why the Chaos forces were present was unknown to all sides although the warnings of the Eldar were seen as giving evidence to them knowing something secret as the animosity between the two forces is widely documented.
They were all about to be proved wrong….
Two weeks after the Chaos incursion the Tau were fighting on two fronts, the planetary Chaos Space Marines had restricted their operations to the Tau held territory and ignored their loyalist cousins. The Space Marines sought to capitalise on this and remove the Tau before cleaning up the small Chaos force that remained. As the conflict between the three sides was continuing to escalate astropath reports claimed that the light of the Astronomicon was growing darker, this coincided with a marked increase in the planetary temperature which spurred on the already lush and verdant world into a period of floral hyperactivity. Within a day of this phenomena appearing the first ships of a Tyranid splinter fleet arrived at the far edge of the system close to the beleaguered Omicron Persei 8.
The truth of the Eldar’s prophecy had now been realised. While ships were deployed to see off the newcomers they were quickly overwhelmed and the planet plunged into another phase of perpetual war. Rather than co-ordinated assaults on their previous enemies each army withdrew and sought to defend themselves against a pervasive Tyranid onslaught.
During any of the running conflicts a small delegation of Eldar often appeared to help out the defenders, why they had chosen this moment to aid the others was unknown although nothing was seen or heard of the Chaos Space Marines once the Tyranids had made planet fall.
At this point it was clear that all sides were in a fight for the planet, Imperial reinforcements had not arrived, most likely due to the Shadow in the Warp and the already tumultuous nature of the Nexus and the Tau were already thinly stretched during the current phase of their expansion. No side was willing to surrender the planet to the Tyranid menace. Thousands of vanguard organisms were destroyed during the first few days of the attack with both continents suffering from constant attack.
This was not the last throw of the dice however, another force appeared suddenly and cleared a path from orbit through to the heart of both defenders camps. These newcomers looked much like the Eldar who had originally provided the warning and proceeded to offered to support the defence effort in exchange for Tyranid bio-matter for “study”. While the Space Marines did not look kindly upon these new Xenos appearing on their world the Tau were more than willing to accept any help they could get.
So it was that the Tyranids ended up fighting against forces of “allied” Chaos Space Marines, Space Marines, Tau, Eldar and Dark Eldar.
Despite the resistance of all the assembled parties the allied forces were quickly pushed back to the eastern continent, it was from here that they would have to withstand the full might of the Tyranid hordes in order to breakout and attempt to reclaim the planet.
However, with the divergent nature of so many of the allied forces, just how long would that alliance last?