For reasons that Imperial forces cannot ascertain, the T'au are very interested in keeping the Imperium from conquering Veren. The planet, a world of glassy black mountains and shallow oceans teeming with corals and algal blooms, is of little obvious strategic value.
Stable Warp-routes from the vicinity of Veren end at the Black Reef, so the planet would be of no use for staging an attack on Tsua'Malor. Nevertheless, the expeditionary force sent to scout the world was immediately opposed by T'au Hunter Cadres attacking in force.
Ebongrave has ordered Veren to be captured and held, but the T'au seem to know the planet very well and use the air-filled coral tunnels beneath its shallow seas to move around Veren with impunity.
The battles through Veren's tunnels and in its mountain passes are vicious and at close-quarters, and the T'au suffer greater losses here than anywhere else in the Canis Salient.
Some in Ebongrave's command believe that the T'au activity on Veren is a huge feint, intended to draw Astra Militarum regiments away from worlds that are truly important, and that the T'au will either leave and force the Imperial Guard to tie up troops garrisoning the world, or draw in more and more until they can wipe them out in some huge T'au-engineered cataclysm.
Such views are near-heretical to Ebongrave's ears, and the war on Veren continues.
- Deathwatch: Core Rulebook (RPG), pg. 356