In a mirror-universe setting like Brighthammer 40,000, the Tau from the regular universe make fine villains with
only minimal modest changes (the same changes the Imperium makes for their anti-tau propaganda).
The Tau were first noticed by the automated Imperial survey probe Olivaw XXIV in M34. As they had just mastered fire, the probe categorized them as "pseudo-neolithic culture". Immediatly after sending the information, the probe was inexplicably lost. An expedition had to stop short of the probe's last known location, as terrible warp storms had enveloped the sector, cutting it off from the rest of the galaxy.
As the storms finally abated in M40, another expedition found the scorched remains of the probe - and the newly forged Tau Empire. In only six millennia, the Tau had been transformed from a peaceful but primitive civilization to a fascist, expansionist, star-spanning empire possessing advanced technology and insatiable bloodlust, making them one of the greatest threats facing the galaxy.
Apparently, a hostile force had come across the young Tau and proceeded to manipulate and mold them into creatures more to their liking. As the Imperial Secret Service had since learned, the Tau Empire rose through the enforced leadership of the Ethereal Caste, which had only appeared after millennia of bloodshed between petty kingdoms.
Further study suggests that blasphemous bio-engineering is responsible for the Ethereals, as they possess the pheromone gland of an insectoid xenos race, which is only known in another galactic arm. This pheromone gland, while already potent in its native species, grants the Ethereal Caste absolute control over other Tau, which they have used to spread their brutal, rigid rule across many other systems.
The secret masters of the Tau are unknown to this day; suspects involve the Exodite Eldar, the Slaan, and the Necrontyr. For now, all that can be done (short of genocide) are attempts to limit and counter the corrupt influence of the Ethereal Tau and their minions.
Tau society is strictly divided into a caste system, supposedly a society of equals, in reality a strict hierarchy.
The Earth Caste are the lowliest Tau, slaving away in terrible conditions to keep the empire supplied with the wargear and supplies that fuel its aggressive expansion. The luckiest of the Earth Caste get to work on miserable farms and live in hopeless drudgery; the unlucky ones are sent to hellish factories, where maimed workers are thrown out in the streets to die.
The Water Caste are derided as conniving and untrustworthy schemers, merchants and diplomats who cheat and swindle those they come across. They mostly prey on too-trusting human societies bordering the Tau Empire, selling malfunctioning and downright dangerous equipment for outrageous prices, then laughing when their customers are killed or crippled after their purchases blow up in their faces. Worse, the Water Caste also attempts to convince human worlds to renounce the Emperor’s benevolent rule for the empty promises of Tau society, bringing in new worlds of unwitting slaves for the Tau Empire. For this reason, the Imperium has forbidden its citizens from dealing with the Tau, and dispatched Special Agents to border worlds in order to protect people from these charlatans.
The Tau’s Air Caste are haughty and cowardly, preferring to rain destruction from far above their enemies rather than engage in a fair fight. They provide pilots and crews for the Tau’s navies and build the starships that allow the empire to expand, but the rest of the Tau view them with distrust at best. The Air Caste are notorious for launching unprovoked and unannounced orbital bombardments on defenseless worlds, laying waste to entire continents for the perverse glee they find in destroying a helpless foe.
The Fire Caste are near the top of the Tau’s social hierarchy. These short-tempered and burly creatures are ostensibly the defenders of the empire, but in reality serve as the backbone of the Tau police state. They spend much of their lives training for combat in vicious live-fire exercises that frequently result in deaths, or else spend their free time bullying the other subraces of Tau. In combat they howl with barely-contained bloodlust, and are known for using prisoners-of-war as target practice, if they bother to take prisoners at all.
But all Tau live in constant terror of the Ethereal Caste, so-named because they appear to be both everywhere and nowhere, an intangible and all-encompassing presence in Tau society. These Tau devote their lives to preserving the order of the Tau Empire, by operating extensive surveillance networks that keep every Tau watching what they say, even when they think they’re alone. If an Ethereal suspects a Tau of treason – or is simply feeling bored – they will order that Tau to kill himself, an order that is instantly obeyed. A paranoid and suspicious group, the Ethereals have a grim, dark view of the universe, and believe that the Tau are surrounded by hostile aliens and beset from within by treacherous segments of their population. They hold that there are no good actions possible in such a galaxy, and instead take the “Lesser Evil” needed for survival. This Imperium has noted with considerable alarm that this philosophy has begun to spread through parts of its own population, driving cynics and nihilists to commit atrocities that they claim are justified by the grim, dark world they live in.
The renegades of the Tau are officially exiled from all Castes. Unofficially, most renegades eventually join the Pale Caste. The Pales are most likely to be encountered outside Tau space, as they are tend to join Rogue Traders or hire up as mercenaries. As people without home, they tend to be extremely diverse. The Pale Caste's members range from freedom fighters who were a mote in some Ethereal's eye, as well as bloodthirsty butchers whose crimes were too great even for the Tau. The caste was created for the actions of O'Shovah (Farsight) and his rescue of a planet on the Tau's farthest most point from the hands of Tau Empire.