T'au Empire - Third Sphere Expansion Septs: The Third Sphere Expansion was begun by the T'au 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 secured at least two new septs for the T'au Empire.During the Zeist Campaign of ca. 999.M41, Imperial Space Marine forces counterattacked against the Third Sphere Expansion, seemingly drawing it to a halt. However, by this time, the T'au Empire had grown to 133% of its size prior to 997.M41, largely as a result of the Imperium's distraction during the 13th Black Crusade and its aftermath in ca. 999.M41.After the withdrawal of Imperial forces from the campaign, the Third Sphere Expansion continued until it ended following the disastrous Second Agrellan Campaign of ca. 999.M41 and increased resistance from the Imperium, which led to the assassination of the Ethereal Supreme Aun'Va.Given the stated size increase of the T'au Empire during this early period, the Third Sphere includes many more septs currently unknown to Imperial scholars. Recognised T'au septs settled during the Third Sphere Expansion include:Ksi'm'yen - The first of a handful of new septs founded in the Third Sphere Expansion, 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 resettled deeper into the T'au Empire to assure their safety and their proper assimilation into the empire. The T'au colonists of this sept seem to be associated with luck, subtlety and opportunistic subterfuge.Fi'rios - The T'au occupying the prime Sept 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 by the wider empire 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 - Mu'gulath Bay was formerly the Imperial Hive World known as Agrellan that was the gateway to the Dovar System. It was the site of a famous T'au victory at the Battle of Mu'gulath Bay, a victory won by both Commander Shadowsun and Ethereal Supreme Aun'Va which had marked the highpoint of the Third Sphere Expansion. After this victory, Mu'gulath Bay was well on its way to being established as a full-fledged sept. However, shortly after the Prefectia Campaign, the Imperium launched a major counterattack against Mu'gulath Bay, determined to reclaim it. In the end the T'au managed to resist, but the Imperium subjected the planet to an Exterminatus action and destroyed it. A small minority of Mu'gulath Bay's T'au population managed to survive the planetary firestorm unleashed by the Exterminatus  thanks to an energy shield dome that was erected by the Earth Caste around a single T'au city, Lo'vash'tau. The survivors were faced with the unsettling truth that they might never be able to travel back to the T'au Empire as their world was now surrounded by Imperial forces. Mu'gulath Bay ultimately had to be abandoned as the entire Damocles Gulf was set ablaze by the Imperial counteroffensive, which effectively separated Imperial space from the T'au Empire. Surviving elements from Mu'gulath Bay then attempted to flee to the nearby Farsight Enclaves, but were set upon by Commander Surestrike on the order of the Ethereals, who were determined that the Farsight Enclaves should not see their strength grow. Surestrike managed to destroy most of the rebel T'au, but some still managed to escape.