Mont'au (“The Terror”) refers to a time when the Tau had fought amongst themselves to the point where disease and famine were so widespread 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