Va'Jiya is a world of deep, dark forests and fetid swamps, and due to its extremely slow rate of rotation its day/night cycle is several standard months in length.
An unusual eco-system has developed around this cycle, in which the forests undergo rapid growth during the long "day," followed by an equally rapid decomposition period during the "night." As the sun slowly rises once more, new shoots sprout forth from the sludge and amidst the skeletons of dead trees and the cycle begins anew.
The dark stinking forests of Va'Jiya are populated by numerous tribes of savage Humans, the descendants of some long-forgotten colony mission that evidently failed. The tribes have developed into separate, highly individual groups that wage a continuous and bitter war against one another.
The tribal warriors are hyper-violent in their methods, and every aspect of their culture is centred upon death and war, from their primitive religion to the construction of their fortified settlements.
Advanced technology has long since vanished from Va'Jiya, although some barely recognisable vestiges of failed artefacts are still to be found in use as improvised weaponry.
Va'Jiyan society has developed a means of controlling its unregulated psyker population, an order called the "Red Lodge" existing apart from the tribes, fulfilling a similar role to that of the League of Black Ships in the Imperium.
The hooded envoys of the Red Lodge visit each settlement at least once in every three day/night cycles and demand that any children who have developed the "Sign" be handed over to them. Where these latent psykers are taken or what their fate might be is not known.
The T'au discovered Va'Jiya and its primitive Human population several solar decades ago, but the Water Caste has been unsuccessful in its attempts to initiate any sort of negotiation with the tribes. Every attempt at peaceful contact has been met with violence, and the T'au are currently considering exactly what should be done about the situation.
As a life-sustaining world Va'Jiya has an intrinsic value to the T'au Empire, yet it does not appear to harbour especially desirable or abundant natural resources.
Very brief studies of the world's unusual flora have turned up some interesting findings however, and so the Earth Caste has petitioned for more expeditionary missions to be undertaken.
The T'au have considered unleashing a large force of Kroot against the tribes in an effort to suppress the savage natives or at least push them back to within a contained area.
The matter is further complicated by the need to keep the Imperium from discovering the tribes of Va'Jiya, for the T'au fear that the hyper-violent natives might be all too willing to rejoin their kin and to fight against the T'au Empire and the Greater Good.
- Deathwatch: The Achilus Assault (RPG), pg. 116