T'au Empire

"Let none doubt that the Tau Empire will bring unity to all – let none doubt that now is our time. Forward, for the Greater Good!"

— Aun’Va, Ethereal Supreme, addressing the Fire caste cadres from atop the ruins of the Agrellan Hive


Head of State:	Aun'Va, Ethereal Supreme
Capital:	T'au
Official Language:	Tau Lexicon
Governing Body:	Ethereal Caste
State Religion:	Greater Good
Major Species:	Tau, Kroot, Vespid, Nicassar, Demiurg
Minor Species:	Gue'vesa (Human), Galgs, Ranghon, Tarellian, Ji'atrix, Hrenian
Military forces Fire Warriors (Fire Caste) Kor'vattra (Tau Navy, Air Caste) Tau Allies (Kroot, Vespid, Gue'vesa)

The Tau Empire is a rapidly expanding alien stellar empire situated within the Imperium of Man's Ultima Segmentum, near the Eastern Fringes of the Milky Way Galaxy. The Tau Empire was founded by the Tau caste called the Ethereals, who lead the Tau Empire in the name of the philosophy they have named the Greater Good. Several intelligent alien races (the Kroot, Vespid, Nicassar, Demiurg and even some humans who have turned their backs on the Imperium, known as Gue'vesa in the Tau Lexicon) have allied themselves with the Tau. The Tau Empire borders the Imperium of Man, and lies within the reach of the Astronomican. It has suffered many raids from the Orks, and also seems to lie in the path of several Tyranid splinter fleets of Hive Fleet Kraken.

History

Tau Empire Galactic Map — A detailed galactic map of the Tau Empire prepared by the Air Caste

Tauspace400 — An astrographical representation of the Tau Empire

TauEmpireMap — A more detailed astrographical map of the Tau Empire following the end of the Third Sphere Expansion

The exact date of the founding of the Tau Empire in the Imperial Calendar is unclear. However the the way in which the Tau were united as a species is a well-known tale. What is known is that only 6,000 standard years ago, in the 35th Millennium, an Adeptus Mechanicus Explorator fleet discovered the Tau homeworld of T'au and determined that its population of sentient xenos were a primitive people at the Stone Age level of development, having only just mastered fire. Since then, the Tau have developed very rapidly to a space-faring species.

Earlier in their history, the Tau were a culture built upon warring tribes. During this time, the Tau's legends tell of the first appearance of the Ethereals at the city of Fio'taun. The fortress city of Fio'taun was under assault by the Tau warriors from the plains. Though negotiation had been attempted, the fierce plains warriors would settle for nothing less than the annhilliation of the city of Fio'taun. For five long Tau years the inhabitants held off the savage assaults with their thick walls and plentiful cannons. However, disease and starvation began to take their toll. As the tide of the siege turned, two mysterious Tau appeared. One made his way into the camp of the plains Tau, exuding a quiet authority that no Tau was able to resist. Soon, the leader of the plains warriors was persuaded to parley with the settled Tau of Fio'taun. Similarly, the other mysterious Tau made his way deep into the fortress. Within a few short hours, the gates stood wide open, and Tau of both sides stood ready to talk.

The Ethereals spoke of the importance of peace and understanding between all Tau. They described a Greater Good that each Tau must strive towards. The besiegers and the besieged quickly agreed with the Ethereals and a truce was reached. Across the world of T'au, Ethereals emerged, each with the same quiet authority and message of harmony and cooperation. With the Tau united, they were able to rapidly develop their civilization's technology, ultimately attaining faster-than-light spacefaring capabilities.

The Tau Empire soon expanded its borders through a series of so-called Sphere Expansions. The Tau Empire has gone through three main phases of expansion as of the turn of the 42nd Millennium. These phases are marked by Tau military campaigns during which worlds and systems are colonised, conquered, or sometimes peacefully persuaded to accept the Greater Good through diplomacy and a manifest demonstration of the benefits provided by advanced Tau technology.

Apart from from the star systems directly colonised by the Tau, which are known as Septs, the Tau Empire also includes the worlds and star systems belonging to the species of the Kroot, Vespid, and the Nicassar. It is currently unknown if the Demiurg are full members of the Empire, allies, or mere trading partners. The Tau Empire is composed of over twenty fully-developed Septs and around one hundred settled worlds, but the exact number and most of their names are unknown to the Imperium.

A known splinter faction among the Tau are the Farsight Enclaves, founded beyond the Damocles Gulf by the Tau Commander Farsight against the orders of the Ethereals. More recently, some worlds and star systems of the Imperium of Man have been conquered by the forces of the Tau, while a handful have seceded from the Imperium and pledged their allegiance to the Tau Empire.

At some point, the Tau sent an expeditionary force to the Imperial planet Malbede where they came into conflict with the Ultramarines Space Marine Chapter in 936.M41. However, the planet proved to be a cursed Tomb World when the fighting of the Tau and the Ultramarines awakened the sleeping Necrons from their tomb beneath the surface. In an effort to combat this terrible threat to both races, the Tau and the Ultramarines combined their forces to defeat the Necrons. Once the conflict was over, the Tau were allowed to evacuate their forces by the Ultramarines Chapter Master Marneus Calgar who proceeded to destroy Malbrede through the use of an Exterminatus order.

In 966.M41, the Tau fought against the Astartes of the Space Wolves Chapter during the War for Kvarium Alpha. On that Ocean World, the Space Wolves' Drop Pods landed deep in the oceans where their occupants, their Power Armour altered by Space Wolf Techmarines to operate in undersea environments, made a move to engage their enemy. On the surface, the battle was fought between the two sides with an equally deadly conflict erupting in the depths of the sea between the Space Wolves and Tau Battlesuits. Thunderhawk gunships armed with torpedoes, prop-bombs and missiles were used to great effect against the Tau's Hammerhead tanks and Manta gunships. Ultimately, the Space Wolves proved to be the victor in the conflict, though hundreds of Tau and Space Wolf corpses had floated to the surface. With their mission complete, the Space Wolves made the long trek back to land across the sea bed.

The Taros Campaign was an Imperial military campaign fought by the 4621st Imperial Guard Army and elements of the Adeptus Astartes to reclaim the Imperial desert Mining World of Taros from the Tau Empire and its Kroot and human (Gue'vesa) allies of the Taros Planetary Defence Force in 998.M41. The campaign was ultimately unsuccessful for the Imperial forces, having taken heavy casualties and losses whilst Taros remained in the possession of the Tau, who renamed it T'ros.

Damocles Gulf Crusade

TheDamocles Gulf Crusade, also called the Damocles Crusade, was the first military conflict fought between the Imperium of Man and the rapidly expanding Tau Empire in the Lithesh Sector of the Ultima Segmentum in the galaxy's Eastern Fringes during the late 41st Millennium. The conflict essentially ended in a stalemate, as the Imperium was forced to conclude its military offensive early to deal with the encroaching Tyranid threat while the Tau sought to begin diplomatic negotiations with the Imperium to show humanity the benefits to be had by accepting the Greater Good.

Before the Crusade, members of the Tau Water Caste had established trade agreements with Imperial worlds on the frontier of the Tau Empire, near the Damocles Gulf region of the Ultima Segmentum in the galactic east, and exchanges of goods and technology were common. Alarmed by the threat of alien contamination, the Administratum readied a suitable response. Almost a century later, the Damocles Crusade smashed into Tau space, destroying several outlying settlements and pushing deep into the Tau Empire. When the Imperial fleet reached the Tau Sept world of Dal'yth Prime, however, the Crusade ground to a bloody stalemate as the formidable numbers and high technology of the Tau and their Kroot allies thwarted every attempt to capture the world or its star system. Many months of terrible fighting ensued with nothing gained on either side. By late 742.M41 the Crusade's commanders eventually agreed to requests from the Tau Water Caste for peace talks. The negotiations were successful and the Imperial fleet withdrew from Tau space unmolested, their reason partly being due to the impending approach of the Tyranid Hive Fleet Behemoth.

Hive Fleet Gorgon

War Without End by MajesticChicken — The Tau repelling the Tyranid swarms of Hive Fleet Gorgon

The Tau Empire was invaded by the Tyranid Hive Fleet Gorgon, a splinter fleet of the much larger Hive Fleet Behemoth, in 899.M41. The Tyranids of Hive Fleet Gorgon were exceptional in their ability to quickly adapt on a biological level to new circumstances of battle, such as evolving immunities to the Tau's energy-based weaponry. This rapid pace of defensive adaptation may have been unique to the Tyranid breeds of Hive Fleet Gorgon, or it may have been a response to conflict with the Tau, who are also a highly adaptable species; albeit on a technological rather than biological level. The combined forces of the Tau and the Imperial Guard destroyed Hive Fleet Gorgon in 903.M41, though only after both humanity and the Tau lost several colony worlds to the rapacious Tyranids.

Tau Empire Member Races

Each alien race within the Tau Empire has a unique and different culture which contributes to the Greater Good in its own way. However, all of the Tau Empire's races hold to their common belief in the Greater Good that binds them all together despite their myriad cultural and physical differences.

Tau

Tau society is centred around the philosophical concept of the Greater Good, where individual Tau sacrifice their personal gain in favour of benefiting the Tau Empire as a whole. It is from this philosophy that the Tau's all-purpose motto, "For the Greater Good!", is derived. The Tau castes, although different in role and organisation, work together for the benefit of all Tau: the Earth Caste provide and produce, the Water Caste communicate and distribute, the Air Caste connect the worlds of the Tau Empire, and the Fire Caste protects and defends all the others. The external image the Tau project towards other species is that they are enlightened, altruistic and idealistic, believing in the unification of all "for the Greater Good".

Tau society is divided into five castes:

Fire Caste - The military ground forces of the Tau Empire
Earth Caste - The engineers, laborers and scientists of Tau society
Water Caste - The diplomats, storytellers, administrators and bureaucrats of Tau society
Air Caste - The pilots and officers of the Tau Navy
Ethereal Caste - The political and reverent leaders of the Tau Empire

Kroot

Kroot society is divided into tribes, known as Kindreds. Each Kindred is an extended family group of Kroot and is led by a Shaper, one Kroot with an intuitive understanding of the DNA absorption process all Kroot possess, and responsible for guiding the evolution of their Kindred. The Shaper, upon identifying a beneficial genetic trait, will instruct his Kindred to hunt and consume that animal, and then control the breeding of the Kindred to ensure that the offspring are born with the selected trait. Often, Kindreds will band together and their respective Shapers will form a Council, of which the most influential Kindred's Shaper will be made leader of the entire group. On rare occasions, a Chief will unite a large number of Kindreds underneath him. Kroot dwell in warm, temperate conditions on the worlds they inhabit, and construct large tree houses from animal hides, bound together by regurgitated dead wood. Some Kindreds prefer to reside in the ruins of the Imperium's Hive Cities on former human worlds conquered by the Tau Empire.

The consumption of flesh is one of the key tenets of Kroot spirituality, as it is believed that the consumption of prey imparts the "spirit" of that prey to the Kroot, which is then passed on to the Kroot's children. This is the average Kroot's understanding of their species' biological ability to undergo rapid evolution, with the belief in "spirit" as a substitute for "genetic absorption." As a part of this belief system, the Kroot believe that when a warrior dies, his spirit should be kept safe by the Kindred. They will consider it their responsibility to consume the bodies of their dead foes, along with their own fallen dead and elders. As one might expect, this custom is not viewed as an "honour" by the human warriors of the Imperium and the Kroot are instead considered cannibalistic savages.

Vespid

The Vespid are an insectoid alien race allied to the Tau who are native to the benighted gas giant world of Vespid. Vespid warriors, called Stingwings, form tactical units referred to as Strains. Each Strain is lead by a Strain Leader. Because the Vespid language is so complex (presumably because of their complex mouthparts) and only the Strain Leaders are given the technological apparatus required to translate their language to their Tau allies, Vespid Strains are very close knit and the Tau military relies heavily on Strain Leaders to help both sides coordinate. Stingwing Strains are highly sought after in Tau Hunter Cadres because of their high level of maneuverability. They are often used by Tau commanders alongside XV8 Crisis Battlesuits, or in scouting roles backing up Tau Pathfinders. In the Tau language, the Vespids are referred to as "Mal'kor," which comes from the Tau words "Mal" meaning "insect" and "kor" meaning "air".

Humans (Gue'vesa)

The Tau Empire hosts a growing population of humans, known in the Tau Lexicon as Gue'vesa, who were once a part of the Imperium of Man. Ranging from ideological defectors who hate the brutality and corruption of the Imperial government to captured Imperial Guardsmen and their children who have known no other life than that among the Tau, they have been fully incorporated into the Empire. While the humans from recently conquered worlds are usually bitter, they do often have Tau supporters amongst them from the start, and the number of these Tau sympathisers only grows larger as they realise the manifest economic benefits that the advanced technology of the Tau Empire can provide. For the most part, the Gue'vesa humans are content and treated well, and are even allowed to continue the worship of the God-Emperor - though the Tau Water Caste moves subtly to discourage this religion and replace it with the Greater Good.

Other Races

The Nicassar are a highly psychic ursine race and were the first alien species assimilated into the Tau Empire.
The Demiurg are a mining species that has allied itself with the Tau.
The Tarellians are dog-like aliens who often work as mercenaries for the Tau.
The Galg are green, scaled, frog-like creatures that often form into mercenary bands which fight for the Tau Empire.
The Ji'atrix are ethereal aliens skilled in void-faring.
The Morralian are auxiliaries that employ Deathsworn to the Tau.
The Ranghon are a race which has accepted the Greater Good and joined the Tau Empire.
The Hrenian are a species that often deploys Light Infantry auxiliaries that aid Tau forces.

Military

Tau empire — The Tau Empire's Fire Caste

Tau ground warfare is carried out almost exclusively by the Fire Caste, whilst the Air Caste is responsible for aerial and space combat, and providing transport between star systems for the Fire Caste. Fire Caste military forces are known as Hunter Cadres, and are similar in size and battlefield role to an Imperial Guard Company. Hunter Cadres are primarily made up of Tau from the same planetary Sept. Auxiliary forces are drawn from the alien races within the Tau Empire, and Hunter Cadres can include Kroot Warriors, Vespid Stingwings, and human Gue'vesa amongst other alien races. The Tau prefer long-ranged combat over close melee combat, disdaining the latter and viewing it as primitive and barbaric. A Tau Fire Warrior's primary weapon is the Pulse Rifle, which can be switched for a Pulse Carbine. A Pulse Rifle fires energetic particles which quickly break down and are projected as a "pulse" of superheated plasma. Pulse Carbines, also carried by Fire Warrior teams, sacrifice range for portability, but incorporates an underslung Photon Grenade Launcher. The Photon Grenades are powerful defensive weapons that not only release a powerful burst of light, but also a sonic shockwave that can temporarily stun the target. As the Tau Empire integrates more and more alien races, they use their unique abilities in their tactics. Often small alien squads will be used as bait to draw enemy fire, though it is important to note that bait squads are not considered expendable but rather dependent on the effectiveness of their cover fire. If Tau forces ever do become engaged in close melee combat, they can call on their cannibalistic allies, the Kroot. The Kroot are expert jungle fighters who appear deceptively slender.

Tau Septs

Each Tau Sept (fully-settled star system) has its own unique cultural identity, but remains wholly integrated within the greater Tau culture. This cultural identity seems to mainly derive from which Tau caste is more numerous and influential in the given Sept. Currently there are over twenty fully-developed Tau Septs within the Tau Empire. Each Tau Sept has its own Sept badge.

Homeworlds

T'au - T'au is the birthplace of the Tau and it is here that the High Council, led by Aun'Va, convenes, its decrees shaping the entire empire. T'au remains the centre of Tau culture and bureaucracy, and produces many Fire Caste warriors. None can rival T'au for prestige, and only Vior'la can match its power.
Pech - Homeworld of the Kroot.
Vespid - Jungle homeworld of the Vespid.

The Spheres of Expansion

Tau Spheres of Expansion — The Tau Empire's Spheres of Expansion

The rise of the Tau can be seen to develop through three distinct phases, periods of intense growth known to the Tau as "Spheres of Expansion". Each of these waves of colonisation is marked by a long building up of resources, after which continual waves of exploratory missions are launched, followed, where needed, by military campaigns to solidify territorial gains. Once a colony transforms itself into a stable settlement, it then serves as a jumping-off point for the next expansion. By the end of the millennia-long First Sphere Expansion, as it later came to be called, the Tau Empire had unfurled across the heavens and consisted of eight fully settled systems known as septs. Named after its prime or "sept world", a sept can include any number of additionally colonised planets or moons, as well as other holdings such as listening posts, sensor fields, shield satellites, orbital cities, and mining operations. Everything is connected, both by a series of space stations and a massive net of communications and sensor relays strung between major locations. Although it might take many generations to establish itself, each sept is unique, with its own cultural nuances and varying proportions of the different castes and alien populations.

First Sphere Expansion Septs

The First Sphere Expansion of the Tau Empire began shortly after their species first achieved spaceflight capability. The Tau homeworld and the First Sphere colonies are of major economic and political importance to the Tau, and form the hub of the Tau Empire.

Tau'n - The first Tau Sept established in the Empire's history, Tau'n, has a chain of enormous orbital docks and controls the largest of the Air Caste space stations; every sept hosts ships of the Tau fleet (Kor'vattra), but none can boast of more than Tau'n.
D'yanoi - Named after the twin moons of its prime Sept world, D'yanoi has survived long isolation due to a space storm of fierce and unnatural qualities. It has also seen many infamous Ork invasions. Having been isolated for such a long period of time from the Tau Empire, its inhabitants are considered somewhat rustic and backwards.
Bork'an - The Sept of Bork'an is a centre of learning and academia, and its system has many rich mining planets. Bork'an has a high percentage of Earth Caste, and Fire Warriors from here are not infrequently outfitted with prototype weapons and equipment.
Dal'yth - Dal'yth Sept was ravaged during the Damocles Crusade; many of its outer colonies and several cities on its prime Sept world were destroyed. It has recovered quickly, thanks to its busy trade ports. Large numbers of aliens can be seen here alongside its famously efficient Water Caste merchants and diplomats.
Fal'shia - Many technological innovations have come from this Sept. Fire Caste warriors from Fal'shia are often the first to try out prototype weapons, armour and system upgrades.
Vior'la - The planet Vior'la orbits a binary star and its name translates as "hot-blooded" in Low Gothic. It is known to produce especially aggressive and skilled warriors. Many Ork invasions have been broken by the sept and the most respected Fire Caste academies reside upon Vior'la.
Sa'cea - The prime world of Sa'cea is the hottest and most densely populated of all Tau worlds; producing more colonisation fleets during the Second Sphere Expansion than any other. Warriors of Sa'cea are regarded as particularly honourable.

Second Sphere Expansion Septs

The Second Sphere Expansion of the Tau Empire began in 018.M39, under the orders of Ethereal Supreme Aun'Wei of the Whispered Wisdom. With new advances in propulsion technology and an already established space empire, the second expansion is marked by more contact with aliens and larger wars. Around 742.M41, the Tau Empire comes into conflict with the human Imperium of Man who launch the Damocles Gulf Crusade in order to keep the Tau's expansion into Imperial-controlled space in check. The retreat of Imperial forces marks the end of the Second Sphere Expansion. Of note are the Septs established by the Fire Caste's Commander Farsight during this period.

Au'taal - Au'taal Prime is a a verdant and beautiful Sept, where only honoured heroes can retire.
N'dras For reasons known only to the Ethereals, this once thriving Sept has largely been abandoned. Its remaining inhabitants are regarded as unstrustworthy, quick-tempered, and of a brooding countenance.
Ke'lshan - Situated near the Perdus Rift, this sept has suffered many invasions and is less trusting of aliens than other Tau. The Ke'lshan fleet and Fire caste forces are only recently back to full strength after their clashes with Hive Fleet Gorgon. Tau from this Sept have grown mistrustful, solemn, taciturn, unfriendly and sometimes openly hostile to alien races.
Elsy'eir - A densely populated Sept with many moons orbiting its prime world, most of which are mined for the valuable ores used in the construction of battlesuits. This Sept is known for its poetry and artwork. Its citizens are considered intellectuals and well-respected for their creativeness by other Tau. The Earth Caste is dominant on Elsy'eir.
Tash'var - A frontier sept, those of Tash'var have been subjected to frequent Ork invasions and pirate raids. As a result, its people have become tenacious and hardy.
Vash'ya - Known as the world "between spheres" the Sept was settled near the end of the Second Sphere Expansion, as it took a long period for the Earth caste machines to make the air breathable. Major Air caste fleets and many defence platforms are docked around the sept world.
T'olku - T'olku is known for its rnany large Ethereal temples, as well as the alien institutes, where many alien ambassadors are brought to be instructed in the ways of Tau culture and society before being assimilated back into their respective homeworlds.

Third Sphere Expansion Septs

The Third Sphere Expansion was begun by the Tau Empire on 977.997.M41 on the word of the Ethereal Supreme Aun'va. The Fire Caste's Commander Shadowsun was placed in command of the expansion, and has secured at least two new Septs for the Tau Empire to date. During the Zeist Campaign of 999.M41, Imperial Space Marine forces counterattacked against the Third Sphere Expansion, seemingly drawing it to a halt. However, by this time, the Tau Empire had grown to 133% of its size prior to 997.M41, largely as a result of the Imperium of Man's distraction during the 13th Black Crusade and its aftermath in 999.M41. After the withdrawal of Imperial forces from the campaign, the Third Sphere Expansion continues to go on to this day. Given the stated size increase of the Tau Empire during this early period, the Third Sphere includes many more Septs currently unknown to Imperial savants. Recognized Tau Septs settled during the Third Sphere Expansion include:

Ksi'm'yen - The first of a handful of new Septs, Ksi'myen is one of the many worlds previously claimed by the Imperium. Those human inhabitants who swore fealty to the Greater Good have been removed deeper into the Tau Empire to assure their safety and their proper assimilation into the Empire. The colonists of this Tau Sept seem to be associated with luck, subtlety and opportunistic subterfuge.
Fi'rios - The Tau occupying the prime world of this Sept wrested it from the grip of an Ork Warlord, and cleansing the star system has proven quite costly. Fi'rios is a Sept whose Air and Earth Caste pioneers are recognised for a tenacious refusal to accept defeat, tempered by a stoic acceptance of the price all must pay in the furtherance of the Greater Good.
Mu'gulath Bay - Gateway to the Dovar System and site of a famous Tau victory, led by both Commander Shadowsun and Aun'Va, Mu'gulath Bay is well on its way to being established as a full-fledged sept.

The Growing Empire

Since the Tau first left the atmosphere of their birth world, their Empire has grown, spreading across the stars in three distinct Sphere Expansions. In addition to the settled systems, or Septs, the Tau realm is rife with all manner of space phenomenon, Tau-made structures and important alien homeworlds. There are also more sparsely populated Tau colonies. Only a few names of such Tau colonies are known to the Imperium:

Arkunasha - The planet of Arkunasha was settled during the Second Sphere Expansion. Despite the aridity of the oxide deserts that made up the world’s surface, the Tau had settled the red planet with a sizeable population. This they had achieved through the tireless work of the Earth caste’s scientists and engineers. By their efforts, the Tau Empire had girded the planet with two necklace-like strings of biodomes that ran around the most temperate latitudes, each a mirror distance from the equator. From orbit, the world appeared as a blood red globe adrift in the ocean of space, banded with two rows of bioluminescent lights that pulsed bluish white like the flanks of some deep-sea organism. In their preliminary investigations, during the earliest expeditions to Arkunasha’s surface, the Earth caste had made a disturbing discovery. Whilst determining the source of the oxide deserts that covered the world, they found a great variance in the metallic composition and even origin of the various particulates. It was as if the world had been entirely covered in metal structures at some time in the distant past, which had long since been utterly ground to dust. Given the depth of the oxide residue, that ancient civilization must have included several artificial cities the size of mountain ranges. Theories abounded throughout the Earth caste about the planet-wide catastrophe that had torn everything upon the planet’s surface apart. Though the Tau had no idea what might have caused Arkunasha’s spectacular death, or who the inhabitants might have been, they had taken their first glimpse of the world-destroying power of the Imperium of Mankind.
Dal'yth - Before the coming of the Tau, the indigo planet of Dal’yth was a wild ecosystem of deep blue foliage and slithering, segmented beasts. It was tamed long ago, during the First Sphere Expansion, and has been brought into compliance with primelevel colony standards ever since. Because of the high proportion of Water caste members upon its surface, Dal’yth has enjoyed extremely beneficial trade agreements, and has recently been counted as one of the Nineteen Wonders of the Tau Empire. Much of the planet’s surface is covered with a tessellating hexagonal net of cities and sub-cities, each connected to the nearest conurbations by a splaying and perfectly regular network of transit tubeways. Clean white magnorail trains whisk the populace to and from, detaching and picking up carriages with slingshot efficiency so that they never have to stop. Though the planet has landscaped hills and even gigantic hexagonal reservoirs dotted across it, from orbit, it looks as if the Tau have settled it with the precision of an Earth caste scientist modelling a new atomic phenomenon.
Es'Tau - Es'Tau was settled during the Second Sphere Expansion.
Ho'sarn - This Tau colony was evacuated in 902.M41 as Hive Fleet Gorgon approached, but the evacuation fleet was ambushed by the Hive Fleet's bioships and completely slaughtered.
Isla'su - The watery homeworld of the Greet, often considered part of the Fal'shia Sept.
Ka'mais - Ka'mais was nvaded by Tyranid Hive Fleet Gorgon in 902.M41. A hidden Necron fleet emerged from Ka'kais' dead moon and destroyed the outnumbered Tyranid bioships, but then continued to land on Ka'mais and harvest the planet's population.
Kor'vattra - Incorporated into every Sept are dozens of major docks for the Tau navy (Kor'vattra), but in addition to these immense stations, manySsepts have also developed vast orbital cities (Kor'shuto ). These can be moved to provide stable "jumping off" points for large research endeavours, military campaigns or major colonisation efforts.
Me'lek
Red Sun Systems - Probes have marked the dense cluster of planets around a string of six distinct red suns; however, the massive Ork population has deterred any further Tau colonisation. The systems are ringed with sensor buoys, in hopes of offering early warning should the Greenskins ever cease their internal fighting and seek to menace neighbouring systems.
Sha'draig - Sha'draig was invaded by the Tyranids of Hive Fleet Gorgon in 900.M41. The Tyranids of Hive Fleet Gorgon were eventually driven off the world, but not before it was almost completely consumed by Ripper swarms.
T'ros - T'ros was recently settled during the Third Sphere Expansion. This is an Imperial world that was willingly annexed by the Tau after opening commercial relations with the xenos empire.
Ta'shiro - Positioned in the deep space between septs are Ta'shiro bases - fortress stations capable of enough thrust to resist drift and maintain permanent interstellar positions. Several patterns of development have been followed in the construction of these enormous floating fortresses - with the largest comparable in population to a continent-sized city.
The Zone of Silence - A devastated region where Hive Fleet Gorgon left behind many barren planets scoured of all life forms.

Non-Tau Worlds

During the First Phase Expansion the homeworlds and colonies of the Kroot and of theVespid were also incoporated into the Tau Empire. Notice that these star systems are not considered septs. In addition there an unknown number of former Imperial worlds within the Tau Empire, some or all of which might still have a human population (see Gue'vesa above):

Pech - The Kroot are the most common of the alien auxiliaries in the Tau Empire, and dozens of Kroot enclaves can be found among the septs. Although they are a far-flung and migratory race, all Kroot eventually feel the pangs that lead them to return to their birth world - the true home of all Kroot Kindreds, the world of Pech. There are eight star systems in the Tau Empire inhabited by the Kroot (including Pech). The Imperium knows only the name of the planet Krath, where the Tau first encountered the Kroot.
Roksh XVI - Site of a secret Tau listening post, the planet was part of the Roksh System that was home to the Rokshashi Wealthweb merchant guild before the entire star system was consumed by the Tyranid Hive Fleet Gorgon in 689.902.M41.
Sha'Galudd - Homeworld of the Nagi, a small species of highly intelligent worms known for their mind control abilities. When first discovered, the Nagi were hated creatures known as mindworms, but since the early violent conflicts, they have agreed a peace accord and joined the Tau Empire. Many Nagi now serve as advisors to the Ethereal caste.
Vespid - Benighted gas giant and homeworld of the Vespid race, this planet is also known for its rich crystal mines.

Artefact Worlds

Three so-called Artefact Worlds are located within the borders of the Tau Empire. As the name indicates, ancient artefacts of unknown origin have been discovered on these planets. It is not known if these worlds possess established Tau colonies.

Arthas Moloch, The artefact world of Arthas Moloch is as grey and desolate as a tomb. Its surface is jumbled with thousands of tumbledown shrines and strange, faceless statues that predate any of the worlds of the Imperium. The world’s surface is cracked and broken, giving the sense that the planet itself died long ago – an impression that is reinforced by the fact that not a single green shoot or patch of moss can be found anywhere on its surface. Not a single living soul makes a home there, though the plaster walls of the planet’s tombs bear the dark brown smears of bloodshed, and ghostly shadows have been burnt into the walls wherever the ruins cluster close together. Though the world is barren as bone, if one possessed of psychic abilities were to behold it with the second sight, it would shine like a gold mine in firelight. The shrine-hold is peppered with artefacts of ancient and mysterious origin, each a priceless wonder left discarded in the dust. Arthas Moloch is also known as a Dead World where Commander Farsight has discovered the Dawn Blade.
Q-15
Landfall

Farsight Enclaves

Although not a part of the Tau Empire, this breakaway faction led by Commander Farsight is known to have settled beyond the region known as the Damocles Gulf, and is named the Farsight Enclaves. These enclaves still believe in the Tau'va, the philosophy of the Greater Good, but operate without the guidance of the Ethereals. The exact names of these Fire Caste-led Septs are unknown to the wider Tau Empire, as the armed fortress stations defending them have proven effective at destroying any probes sent to the Enclaves. When sighted generations later, these forces and fleets bore markings similar in design to those used by the Tau Empire, but in colours and patterns never sanctioned. Like any distant colony, much of the equipment and armour used by those within the Farsight Enclaves is slightly dated -- the equipment most prevalent at the time of Farsight's disappearance. There has been, however, unsettling evidence of classified technology and recent Tau prototypes present within the Enclaves. Time will tell whether this is the result of spycraft, theft or defectors from the Tau Empire who have been aiding those within Farsight's domain.

Sources

Apocalypse
Battlefleet Gothic Magazine 17, "Tau In Space," & "The Tau Empire," pp. 6-17, 25-27
Codex: Space Marines (5th Edition), pg. 48
Codex: Tau Empire (6th Edition)
Codex: Tau Empire (4th Edition)
Codex: Tau (3rd Edition)
Codex: Tyranids (5th Edition), pp. 18-21
Deathwatch: The Achilus Assault (RPG), pp. 109-130
Deathwatch: Core Rulebook (RPG), pg. 315
Deathwatch: The Jericho Reach (RPG), pp. 56-63, 66-67, 69-71, 91-95, 98-99, 104-106, 108
Deathwatch: Mark of the Xenos (RPG), pp. 8-31
Deathwatch: Rising Tempest (RPG), pp. 5-13, 15-17, 118-119, 123, 130-131, 139-140
Farsight Enclaves - A Codex: Tau Empire Supplement
Imperial Armour Volume Three - The Taros Campaign
Planetstrike (5th Edition), pg. 57
Warhammer 40,000 Rulebook (6th Edition)
Warhammer 40,000 Rulebook (5th Edition), pg. 129
Warhammer 40,000 4th Edition Rulebook, pg. 128
White Dwarf 317 (UK), "Eavy Metal: Heroes of the Tau Empire"
White Dwarf 316 (UK), "Index Xenos: Vespids"
White Dwarf 315 (UK), "Battle Report: Death on Kol’n - Tau vs. Dark Eldar"
White Dwarf 314 (UK), "Tau Empire: Preview"
White Dwarf 262 (US), "Index Xenos: For the Greater Good, an Investigation into the Newly Discovered Tau Empire"
White Dwarf 261 (US), "Tau: A preview of the new race for 40k"
Last Chancers (Novel Series) by Gavin Thorpe