Dogged defence on Tyranteous

Although the tau had all but defeated the Imperium on Tyranteous, at the beginning of 11.013M42 Luthor's order to defend had been followed by General Cunningham. Defences had been built in the two remaining imperial footholds left on the planet. Never the less the tau were keen to finish off Tyranteous to focus their efforts on Zadoc. At the beginning of 11.013M42, despite the defensive preparations taken by the imperium, the tau showed no sign of slowing their offensive.

Commander Skyfall's relatively fresh troops now took over the main thrust, giving those of Windgather a chance to rest. Initially the new tau advance stalled as the ultramarines launched a series of well planned and well executed raids against the aliens, bringing their offensive to a halt. However the ultramarines were few in number and gradually the tau superiority in numbers began to tell in the east, as Skyfall launched probing attacks, feeling out the imperium's weaknesses.

The tau commander felt he had found such a weakness in the far south of Grent. He managed to find an area more weakly defended and soon broke through. On 0211.013M42 the Librians counter attacked, supported by the Blood Martyrs chapter of space marines. The astartes immediately closed with the tau, ordering the Librians to cover their advance. In the ensuing action a number of incidents occurred which aroused suspicions of the loyalties and willingness to carry on of the the Librian regiments.

During the battle it was noted that at least one Librian tank, a Librian Leman Russ Executioner variant, fired upon and killed four Blood Martyrs at close range, claiming it was accidental. This on its own would have been put down to battlefield confusion, but moments later a Manticore artillery strike "accidentally" shelled a Blood Martyrs' Land Raider, instead of the enemy, in a show of astounding inaccuracy (which the Librian artillery corps has been trying to shake off for years) or something much worse.

After the battle several more incidents of "friendly fire" came to light, and the overall effect of these certainly blunted the Blood Martyrs' assault, allowing the tau to whether the astartes storm and advance into east Grent. His line compromised Cunningham withdrew north to prepared defences in front of Yellowmouth. The situation remained grave, and now there were suspicions that Librian regiments were trying to defect to the tau. Given that Libria was now the only secure base of operations in the Zadoc subsector, this was worrying news for the Imperial high command, and Lord Titus Luthor dispatched inquisitors and the commissariat to investigate the situation.