Vallia is, were it under Imperial classification, a Death World, given the abnormally aggressive nature of its flora and fauna. It has been declared out of bounds to all but those with the very highest level of authority.
The T'au however have far less experience with such worlds, and have yet to fully appreciate the extremes of malevolence that worlds like Vallia can exhibit. The Explorators and Rogue Traders of the Imperium know that Death Worlds are more than just dangerous environments, a fact that the T'au are only just beginning to understand.
Vallia is almost entirely swathed in jungle, the flora extending to every corner of the surface and every crack below it. Even the seas are choked with carnivorous plant life, as are the lightless caverns deep beneath the ground.
In fact, Earth Caste scientists have come to believe that the world's plant life is dominated by one single organism, its roots extending deep into the world's crust. In places, roots that must be hundreds of metres in circumference have destabilised the underlying tectonic processes, causing hundreds of volcanoes to erupt across the surface.
Numerous T'au expeditions to Vallia have ended in disaster as the plant life as well as animal life has reacted in concert to expel the invaders. The first three expeditions were completely annihilated, but not before the discovery of unique and highly desirable natural resources in the form of compounds excreted by the carnivorous flora.
A fourth expedition was dispatched to confirm these findings, this time accompanied by a Fire Caste battle cadre entirely equipped with Battlesuits configured to hold the predatory flora and fauna at bay. Although dozens of Fire Warriors died in this mission, samples were recovered and shipped back to Tsua'Malor for analysis.
What was discovered during that analysis caused a paradigm shift in the way many Earth Caste xeno-biologists view the evolution of complex symbiotic biospheres, a view which has yet to be accepted by those who have not seen the evidence with their own eyes.
The Earth Caste scientists of Tsua'Malor have discovered that every life form on Vallia is interconnected. This symbiosis goes far beyond the conventional interconnection seen in many life forms, and extends to an amalgamation of will and a form of gestalt intelligence.
Though they have yet to fully accept their own findings, the T'au have discovered that the Death World of Vallia is in fact sentient, ruled over by a single, utterly malevolent intelligence that can mobilise every single living cell on the planet to repel any who would set foot in its domains.
As if this realisation were not shocking enough to the Earth Caste's world view, some amongst their number have advanced the notion that this planetary sentience can be somehow reasoned with, perhaps even convinced of the primacy of the Greater Good.
Initial Fire Caste estimates have placed a high possibility on any T'au contact force being completely wiped out, and the Water Caste can see no way in which meaningful discourse could be established. Nonetheless, research continues and a future contact mission is a distinct possibility.
- Deathwatch: The Achilus Assault (RPG), pg. 117