Fourth Sphere of Expansion
In the aftermath of the Second Agrellan Campaign, the Tau faced a difficult strategic situation. The path to continue expansion through Mu'gulath Bay was blocked, and to the galactic east was a sea of dead space thanks to Hive Fleet Gorgon. To the west, the Necron Sautekh Dynasty reigned supreme. It was at this moment of crisis that the Great Rift formed, further exacerbating the Tau situation with vicious stellar storms that killed millions.
Ultimately, the Ethereal Council decided to renew the Tau's expansion with a dangerous plan. The AL-38 Slipstream Module was developed after many years of studying Imperial FTL technology. The new technology allowed for the Tau Fleet to move at incredible speeds and a new Sphere of Expansion was declared under the command of Commander Surestrike. Surestrike's armada gathered at Numenar Point, but due to the sheer size of the fleet to utilize Slipstream technology disaster struck. The multiple disruptions in reality caused by anti-matter fields created a chain reaction that created a massive wound in realspace. The rift swept across the Tau fleet, devouring it whole. The disaster was broadcast across the entire Tau Empire and the Ethereals scrambled to contain all knowledge of the catastrophe.
However, the Fourth Sphere was not entirely destroyed. Cast into a nightmarish dimension, 3/4ths of the fleet was destroyed by alien creatures not yet fully understood by the Tau. Saved from oblivion by a many-armed entity within the Warp believed to be a possible God created as an echo of the Tau race, the survivors eventually emerged in the Chalnath Expanse deep within the Imperium of Man. After many years, contact through the rift was reestablished using communications Drones. The Wormhole was dubbed Startide Nexus and was used to launch the Fifth Sphere of Expansion. The initial reemergence area of the Fourth Sphere has since been fortified into the Nem'yar Atoll.
What became of the Fourth Sphere's Auxiliaries has since become a mystery. However rumors speak of mass expulsions and executions directed at any non-Tau within the surviving fleet. It has been theorized that this uncharacteristic action by the Tau was initiated after their trauma of what they had encountered within the Warp, somehow blaming the alien races for the catastrophe.
Survivors of the Fourth Sphere continue to have a dark reputation. Its contingents display a brutality and xenophobia uncharacteristic of the Tau. Tau of the Fourth Sphere believe that the abomination that saved them within the Warp was formed from the auxiliaries psychic perceptions of the Greater Good, and can only be slain with their extermination. During the Fifth Sphere, Human prisoners were massacred and Auxiliaries were intentionally sacrificed in battle by Fourth Sphere survivors. The Massacre of the Dul'un Lakes and the Eight Days of Infamy are the most famous episodes from this controversy. Eventually, Shadowsun and the Ethereal Council disciplined the Fourth Sphere commanders with a ritual punishment known as the Malk'la before they were returned to the ranks or sent back to the empire for re-assimilation. After a massacre and bloody Kroot uprising upon the colony of Ky'san, all auxiliaries were removed from contingents of Fourth Sphere Tau.
- 1: Warhammer 40,000 8th Edition Rulebook pg. 134
- 2: Codex: Tau Empire (8th Edition) pgs. 22–25
- 3: War of Secrets (Novel) Chapter 17