Per Usual: Images and links are from Lexicanum or the Warhammer 40k wiki 

The Tau are a youngish race, at least by galactic standards who have formed a multi-race empire on the edge of the galaxy.  Scientifically inquisitive they are starting to bypass the Imperium of Man in technology.  They are more interested in doing their own research and advancement than just copying everyone else in the galaxy.  The Tau or T’au, are a race of blue skinned bipedal aliens who have embraced the idea of the greater good as their core philosophy.

Or they were forced into it.  Tau society includes other races such as Kroot, Vespid, Demiurg, and even humans, more on that later.  I have included the story from Lexicanum so you can get an idea of what happened and how the Tau came to embrace the way of the greater good:

Mont’au (“The Terror”) refers to a time when the Tau had fought amongst themselves to the point where disease and famine were so wide spread that the species, itself, was threatened with extinction. (This was late in the 37th millennium by Imperial dating)
The Ethereals of Fio’taun tells the story of the coming of the Ethereals and the start of Tau’va. On the plain of Fio’taun, an alliance of air-Tau and plains dwellers sieged a city-fortress of the builder-Tau for five seasons. The alliance ignored negotiations attempted by the trader-Tau. One night a Tau of “unusual appearance” walked into the alliance’ camp and commanded the sentry to escort him to the leaders. At the same time another Tau, of the same manner, appeared within the walls of the city-fortress and also commanded to be taken to the city leaders. Within an hour, the gates of the city-fortress were opened and, led by the strange Tau, the city leaders met with the leaders of the alliance. These strange Tau called themselves Ethereals. The Ethereals spoke of how the talents of each Tau could be harnessed for the Greater Good if only they could put aside their wars and work together. By daybreak, a truce was agreed to by both sides.
This was the beginning. Soon more Ethereals emerged and peace spread throughout the planet. Tau now worked together to build a society that benefited all Tau.¹

Basically these never before seen Tau show up who have stones imbedded in their foreheads and suddenly everybody are friends.  And NO ONE QUESTIONS THIS, this would be like Trump Maggats and the Rainbow Coalition working together to improve the environment and boost the economy at the same time, with NO fighting.  The theory is that the Ethereals emit pheromones which suppress the other Tau and make them subservient.  There are some Tau who have started twigging onto this and no longer meet with Ethereals with out some sort of atmosphere filtering equipment.

The Ethereals then divided the population into Castes, each Tau knowing their place, their duties, and their social heirarchy.  Below is a list of the castes and also the aliens associated with the Tau (from Lexicanum):

The Castes (in no particular order):

• Air Caste
• Earth Caste
• Fire Caste
• Ethereal
• Water Caste
• Alien Auxillaries
  • Demiurg
  • Gue’vesa
  • Kroot
  • Nicassar
  • Vespid

The Gue’vesa are humans descended from Imperial Troops left behind by the Imperium after the Damocles Gulf Crusade, given the choice to join or be imprisoned, they joined, FOR THE GREATER GOOD.  Of course the Imperium considers them traitors and will focus on them during any battles in which they are engaged.  Want to see an Inquisitor explode in rage, let them know of a planet of humans who are part of the Tau Empire, and faster than you can say Torquemada, a war fleet or an Exterminatus ship.  Astartes will immediately ship out if they find the location of one of these planets.  No human should live in any sort of happiness or comfort, unless it is under the Ecclesiarchy’s thumb.

The Kroot, holy shit, these are some murderous, vicious assholes who practice genetic engineering by eating the dead of their enemies.  They can also gain some of their enemies memories by eating the brains of their dead enemies.  It does not always work and they may actually devolve into 4 legged Kroot hounds who have lost most of their ability to think but gain heightened senses.

I will get into more about the Tau later, including their armies and their weapons, which are really cool as they have rail guns, RAIL GUNS FOR THE GREATER GOOD!

Personally I think the Tau are really cool, really cool.  They have the largest and most expensive model, a space ship that contains an entire army.  Plus the models are really cool, I am hoping they get an update soon.

Till next week!