The Serpent Rises

The Serpent Rises is the first known recorded Imperial campaign conducted by the strange and foreboding Mortarchs Serpentia Space Marine Chapter against unknown xenos corsairs who made the mistake of attacking an allied Imperial fleet. The then-unknown Chapter struck like lightning and without warning, slaughtering all those that would dare attack loyal servants of the Emperor. Following the massacre of the xenos corsairs, this unknown Chapter disappeared just as mysteriously as they arrived.

History
An Imperial fleet returning to friendly space to resupply and repair, is ambushed by xenos pirates. The beleaguered and depleted Imperial forces are swiftly overpowered, and enchained to be taken away to some horrific fate. However, when the xenos return to their ships with their prizes, they find the decks deserted, thick smoke obscuring all and shadows dancing in the light lost gloom. Then it came, softly at first, distant, like a noise you think you heard but then can't seem to affirm, but then louder, and louder, a dry, bone chilling rattle, echoing from everywhere and nowhere. Shadows begin to form in the smoke, but when the xenos open fired, nothing was there but swirling mist. It was then, as the xenos began to turn on one another in fear and desperation, that they struck.

Stalking from the darkness, bolt guns booming and chainswords roaring, and all the while, the deathly rattle echoed around, the only sound made by those horrific marines. Within hours, every Xenos pirate lay dead, and their prey was released. Those survivors told of marines in armour as black as the abyss, with trim like a sun-bleached bone, though of which chapter they were, none of their markings were seen through the dense smoke. The Marines left without a word, and the smoke left with them.