Orbiting a large red star on the absolute edge of the Tau Empire's influence, the sept of Trae'Shan is most notable for its adoption of several aspects of the culture of a local alien species newly admitted into the Empire, albeit with much of it removed.
The eponymous prime world of Sept Trae'Shan is a dense world rich in minerals, large in size and freezing cold in climate. Vast mountain ranges dominate the 8 continents, and the sky is colored a dull grey. The winds are harsh, and night settles abruptly with little warning. Tau settlement is highly focused into several cities/mines burrowed into mountainsides, connected only by a currently underdeveloped rail system that is often in need of repairs.
The lands of Trae'Shan is one hostile to life and harsh to endure, though Tau ingenuity building upon the customs of the Rak'Shun have proven more than capable of countering the many dangers present on the world.
Perhaps the first thing anyone would notice upon setting foot on the world is its heavier than normal gravity. While an un-augmented Human or Tau can traverse the planet without worry of injury it renders their movements sluggish and cumbersome, which translates to death in a world in perpetual winter and crawling with ravenous beasts. The natives have evolved a short lean stature as a result, as well as developing a wide variety of physical training regimens that perpetuate their daily life as well as having importance in their beliefs. The Tau have adopted such physical practices (minus the associated spiritual beliefs of course) and so Tau of all castes from this Sept tend to be mildly more muscular than average, and have greater familiarity with hand-to-hand combat and melee weapons (skills which almost never see actual see actual use outside of ritual and situations where persons must be subdued alive and without harm).
When the Tau set upon the world of Trae'Shan they found it already inhabited by another species, the Rak'Shun. Planet-bound and barely into their own industrial age, the Rak'shun are split into many clans that each live in a large "den" carved into a mountain or in a cave system, yet they share the same religion, an odd eclectic mix of beliefs that include ancestor worship, high valuing of the ideal of brotherhood, and reincarnation. While their strong attachment to family is a concept alien to the Tau, their faith in brotherhood and working as a whole was easily compatible to the Greater Good. However, their kinship only extended to the clan level, and there are a number of clans that are reluctant to cooperate with their former enemies. The Tau are confident that in time all clans will come to see the wisdom in abandoning their old feuds and working as one, but just in case there is always the Fire Caste if the need arises. As noted before, the Tau of Trae'Shan have adopted numerous aspects of the local religion- Rak'Shun temples have been repurposed for meditation and "enlightenment", the old journey-paths now used for the Tau to build physical endurance as well as reflect, and the Greater Good now seems to be more mystical to the Trae'Shan Sept, firmly believing that the Greater Good is the natural destiny of the Galaxy, as if it enjoyed some sort of cosmological mandate.
As with much else of Tau culture, the Rak'Shun religious system is heavily inspired by East-Asian religious beliefs and practices, specifically Confucianism and Buddhism. Hopefully there will be time to flesh it out more in the near future.