What Is Dead May Never Die

What Is Dead May Never Die was an Imperial xenocidal purgation campaign conducted by the merciless Mortarchs Serpentia Space Marine Chapter against the upstart T'au Empire upon the Imperial world of Nimbosa during the Nimbosa Crusade.

History
The planet of Nimbosa, having seen some of the bloodiest and most ruthless Tau engagements, to the point where the xenos Commander, Brightsword, is censured and returned to Tau, once again fell into the hands of the crusading xenos. Powerful defence batteries and fortifications are swiftly erected in an attempt to secure the planet for the Greater Good. Imperial resistance is still present on the ground, but the tide has been steadily swayed in the Tau's favour. As all hope for the Imperial resistance seems to be dwindling, a strange storm formation builds in the planet's northern mountains, and a thick fog rolls down its slopes in a wave of grey smoke.

Tau entrenchments are soon blanketed in the thick layer of mist, destroying vision and leaving night vision tech all but useless. It was then that advanced target-finder systems employed by the Tau began to sound. Multiple units were appearing, and disappearing in the radar augur, with no sign of actual foes appearing from the mist. As Earth Caste engineers frantically try to fix what they believe to be faulty tech, it began -- the rattling, chilling noise that seemed to echo not just from within the mist, but from the very walls themselves.

The Fire Caste warriors, already on edge, is only kept in line by the stern guidance of the present Ethereals. After days of echoing bone rattle cries, they came, stalking from the gloom like unearthly spectres. Space Marines in black and bone plate, walking without urgency or fear. Tau gun batteries opened fire in a fusillade of deadly ion and pulse, but the advancing marines seemed hardly to slow. Shots would strike true what was thought to be a foe, only for the image to shimmer away as only a shadow in the gloom.

Even those blows that struck home seemed to hardly faze the indomitable marines. Limbs were blown from bodies and rending holes blasted in the torsos of these Astartes, but the jubilant Fire Warriors would only have a moment to celebrate before the assumed dead marine would simply shrug off the wound and continue forward. Tau forces could not halt the implacable march of the shrouded marines, and within days, a full scale retreat was sounded. Though ultimately the planet would change hands several times in the future, forevermore, that Fire Warrior cadre has feared the coming of the fog.