The Assault on Cephian IV was an Imperial military action that took place in the 41st Millennium on the Feral World of Cephian IV, where the Black Templars Space Marine Chapter had maintained a Chapter keep in its northern chain of mountains for over four thousand standard years, following the liberation of this world from the warriors of the Alpha Legion in the early 37th Millennium. This attack was the culmination of a dastardly alliance between the deluded followers of the Ruinous Powers and the depraved xenos known as the Dark Eldar, intent on murder and torture.
A hot, arid world, Cephian IV had little to recommend it save moderate mineral deposits and a fierce warrior culture that demanded total ruthlessness amongst its soldiers. A seditious Chaos sect fostered by the Alpha Legion had taken root amongst its warrior cults and rebellion was soon engulfing the world, a rebellion mercilessly crushed by the zealous Space Marines of the Black Templars in 343.M37. With the treachery over, the Chapter's Marine-Artificers and an army of Servitors constructed a grim and imposing fortress that served to remind the planet's inhabitants of the dominance of the Emperor of Mankind and as a staging post for the Crusades into the Ghoul Stars and beyond. The Chapter keep was also used as a recruiting base, for the people of Cephian IV came from a fierce martial culture and provided many Neophytes for the Black Templars. And, far from resenting the Black Templars' continued presence, the warriors of Cephian IV respected their strength and harshness. Though over four thousand standard years has passed since the fortress was raised, it has always been occupied by at least one Crusader squad and legends tell that should the keep ever be abandoned or fall, it would spell the last days of the Chapter. But, over time, such respect turned to hate and cults once again flourished within the ranks of the warriors of Cephian IV and, though outwardly the planet remained loyal, worship of the Dark Gods took hold -- once again promulgated by agents of the Black Templars' old enemies, the Alpha Legion, who had designs and reasons of their own to foment revolt on the world. In addition, diabolical pacts sealed with the Dark Eldar helped to distract their ancient enemies from mustering to intervene in their grand designs.
The current Castellan of Cephian IV is Chaplain Develain, a warrior of great skill and wisdom who has served the Chapter for over 200 standard years. His experience in battle is passed to those Initiates and Neophytes who muster at Cephian IV and none can doubt his loyalty or devotion to the Emperor and the Chapter. When the warriors of Marshal Ulliquel's Thunder Crusade entered orbit over Cephian IV, they demanded fresh Neophytes to join them in crushing a spate of Chaos Cultist uprisings in the Dominion of Storms. The Neophytes deemed ready to join the Crusade by Chaplain Develain, the current Castellan of Cephian IV, were pushed harder than ever before to prove themselves worthy of such an honour in a last trial by fire.
Unbeknownst to the Black Templars, Chaos Space Marines of the Alpha Legion had formed an unholy alliance with the Dark Eldar from the Kabal of the Sundered Blade in order to delay or prevent the Thunder Crusade from reaching its ultimate destination -- the Alpha Legion for reasons too impenetrable to discern, the Dark Eldar for the simple pleasure of inflicting pain and misery on any fresh meat they might capture.
After successfully completing their last trial by fire deep in the Harake Mountains, the surviving Neophytes were being escorted back to their transports by Initiates of the Chapter keep. While en route to their Rhinos, a force of Dark Eldar launched an attack on the Black Templars in order to capture the new recruits as slaves and slay the master of the Chapter keep. The Black Templars fought a desperate battle to protect their Neophytes. Chaplain Develain and his warriors escaped the ambush, but while en route to the Chapter keep aboard a flyer, a missile streaked up from the mountainside and exploded beneath the aircraft's engine compartment. The pilot had time for only a brief distress call before crashing in the foothills of the Harake Mountains, only a few miles from the Chapter keep.
A rescue mission was immediately despatched to rescue any survivors and recover the warriors' relics and gene-seed, but even as the Black Templars closed in on the crash site, it was clear that they were not alone, as a shambling host of mutants, Heretics and Traitors attacked the survivors. The enemy had revealed its hand too early with the attack on the downed flyer and the Black Templars readied themselves for a full assault. Soon enough, a vast host of enemy warriors, Heretics and mutants advanced on the Chapter keep led by Chaos Space Marines from the Alpha Legion and a dangerous rogue psyker of terrifying power. For days, the Forces of Chaos battled the Black Templars, dying in their thousands as the guns of the Chapter keep scythed their enemies down. Chaplain Develain knew that he had to end the battle quickly, and resolved to lead a strike force into the heart of the enemy to slay their leader.
During the ensuing battle the Castellan slew the rogue psyker while Emperor's Champion Marquand single-handedly took on a Chaos Defiler. The valiant warrior danced past the claws of the clumsy Defiler and rammed his Black Sword upwards, into its belly, utterly destroying the daemonic spirit within. Howling in pain, the Defiler crashed to the black-ichor soaked ground, a shattered wreck, utterly destroyed. The Black Templars displayed their typical fortitude and tenacity in the defence of their Chapter keep, driving the Forces of Chaos into retreat. Thanks to the unreliability of their firearms, the mutants had eliminated a great many of their own number themselves, forcing the remaining wretches to flee before the might of the Space Marines. The Black Templars showed no mercy to those that had somehow survived their final counter-attack, slaying any and all survivors.
- White Dwarf 311 (US), "The Assault of Cephian IV" by Graham McNeill, pp. 24-49