Hey all, not sure if this is the best place to post this but here goes. I have a Tau force which I am currently painting and a Marine force which I will be starting. Inspired by a BOLS post a few days/weeks ago here is my attempt at some background for my Tau, and as a bonus it ties in with some background to my chosen marine chapter the Silver Skulls. I have looked at both the Tau and Marine codexes so this 'fluff' should be accurate and fit the 40K universe history and time frame.
Apologies if it a bit long. Any feedback at all would be appreciated. Anything you'd change?
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The Psamathos Sept:

The Second Sphere Colony of N’dras was voluntarily abandoned at the instructions of Aun Seki’yl Esta (Ethereal Ever Right Soul) almost half a century before the Third Sphere Expansion began; no reasons for this were ever given to the Tau commanders. The Tau on the planet were all transferred to a highly mobile space fleet and have since become masters at void combat and counter-attacking against incursions into Tau space.

In 997.M41 the Tau Empire’s Third Sphere Expansion began in earnest. With Commander Shadowsun in overall command Tau forces conquered large swathes of Imperial territory. This was in part due to the stripped defences on the Imperial frontier but mostly down to Shadowsun’s tactical prowess and the newly raised Tau forces from Second Sphere Colonies, including the N’dras Sept.

The N’dras fleet containing some of the most experienced of the Tau Sept forces and led by Shas’O N’dras Kart’yr (Commander, N’dras Sept, Space Bound) entered the Psamathos system in early 998.M41 and encountered resistance of magnitude on the Imperial planet of Psamathos IV.

A large planet rich in mineral resources but with a low level toxic atmosphere, Psamathos IV was populated with just under 1 billion humans. These mostly lived in the seven hive cities, and up to 83% of the individuals were directly involved in mining operations before the war. The planet supplied the nearby Augura system and the Forge World of Aseptoch. Psamathos IV’s elite resided on its three moons (Norns, Moirae and Parcae) all with high agricultural resources and hospitable atmospheres. When the N’dras fleet arrived they found the Imperial forces woefully unprepared.

After sweeping aside the few Imperial vessels left to guard the region, the smallest of the Tau craft were sent to pacify any Imperial outposts situated on the other five planets of the Psamathos system, including a Mechanicum research facility. The vast majority of the Tau forces fell upon the moons of Psamathos IV quickly overcoming the scant resistance, Earth Caste Tau then wasted no time in constructing urban centres and the population of the moons was well under way as combat continued on the planet below. War on Psamathos IV was not to prove as simple.

Tau forces encountered brutal resistance on the planet especially when attempting to storm Pesadae Hive, the rich mineral deposits and mountainous terrain of the planet further hindered the Tau as communication links were disrupted and commanders struggled to maintain coherency. The Tau soon settled into a pattern of swift mobile attacks at any vulnerable Imperial positions while avoiding the Hives until necessary. Only in 999.M41 did Tau forces finally capture Pesadae Hive and make any significant headway in the remaining six. Though gains were made militarily against four of the hive cities, the tau’s greatest success was through the Water Caste envoys sent to the cities of Lopero and Justag; the poorest and most oppressed Imperial citizens reacted favourably to Tau propaganda and began to desert the hastily created PDF regiments and join the Greater Good. However Tau successes were not to continue as Imperial forces led by Captain Sicarius of the Ultramarines smashed into newly conquered Tau space all across the Third Sphere, the Zeist Campaign had began.

It was the Silver Skulls chapter who led the assault in the Psamathos system, with forces from the Abraham III, IV, and XX guard regiments. The Tau fleet was scattered in the face of overwhelming odds, and again the moons of Psamathos IV were quickly taken to use as staging posts for the planet-side invasion, though a few Tau pockets held out. This time it was the Tau who were able to use the planets minerals and terrain to thwart the invading army. Imperial gains were slow and losses were far above expectations. The Tau had learnt their lessons well, with N’dras forces becoming expert city fighters and masters of hit and run attacks. Nevertheless the sheer strength and size of the Imperial campaign caused the Tau to fall back and also begin to arm and use human auxiliaries. The Silver Skulls attacked viciously wherever these human traitors were found and more than once were lured into an ambush because of this. Then in late 999.M41 the Silver Skulls suddenly left the planet, the Imperial Tarot was predicting dire times for the empire of man and the chapter was never one to ignore an augury.

The Imperial Guard also began to withdraw and leave the system but only 80% had left the surface when Exterminatus was unleashed. Whether this was ordered by the Astartes to spite the Tau, punish the traitors or simply due to the more mercurial predictions of the Librarians is unknown, what is certain is that the planet’s biosphere was ripped apart and all those souls caught outside of the cities were annihilated. Only those who had been in the deepest part of the cities and underground bunkers were spared, and these were few.

The N’dras fleets have since returned to find that less than 8% of the original N’dras Sept survived the war, either through being lucky enough to avoid the worst of the Exterminatus or through being engaged upon the moons or with the fleet. The death toll for the human population was catastrophic, and that the Imperium would destroy their own loyal subjects who had defended the planet for so long is an enigma for the Tau and is now a leading factor in the Sept’s continued war against the Imperium. The Sept has since been renamed the Psamathos Sept, and the moons are currently being rebuilt as Tau centres. The planet will be mined by specialist Tau teams and the resources used to strengthen the Tau cause. Yet the greatest asset gained by the Psamathos Sept is not in minerals or finally achieving a new home planet, but in the acquisition of Gue’Vessa (human auxiliaries) who have been summoned to the side of the Tau mostly due to the callous actions of the Silver Skulls, and the knowledge that further human’s can be won over to the Greater Good. But as 999.M41 draws to a close, the fate of Shas’O N’dras Kart’yr is still unknown, presumed dead, the Tau are now led by a coalition of suitable replacement commanders awaiting one to finally make a name for himself and achieve overall command.
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Thanks in advance for any response, good gaming to all