The Ethereal Caste of the Tau, known as the Aun in the Tau Lexicon, are the ruling caste of Tau society and direct the other four castes in their pursuit of the Greater Good of the Tau race. They are the ultimate rulers of the Tau Empire, which is led by a council of the wisest Ethereals. The Ethereals guide the Tau race as a whole to work for the Greater Good of all Tau and their allied races, and decide who is worthy enough to be absorbed into the Tau Empire. Their power over their fellow Tau is such that were an Ethereal to order another Tau to die, that Tau would do so quickly and gladly. The Imperium of Man would give much to learn the secret of the Ethereals' power to command their fellows. Ethereal Caste members are occasionally seen on the battlefield, but whether they are leading or merely observing is unclear.
The Ethereals first appeared in Tau history when the Tau were in an age of turmoil known as the Mont'au towards the end of the 37th Millennium. It was the Ethereals who ended the infighting between the Tau cities and tribes on their homeworld of T'au at this time and united them in pursuit of the Greater Good. It is believed by some Imperial scientists that the Ethereals control and ensure the loyalty of the other Tau castes by emitting certain pheromones.
- Codex: Tau Empire (4th Edition)
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