The Bachramshedannan Crusade

It is the year 012.M42. It is the age of the Dark Imperium, and the Cicatrix Maledictum has torn the galaxy in two. This however, is not of great concern to the myriad parties that have assembled over Bachramshedannan, far from the galaxy's fractured core. They seek only the secrets hidden beneath this mysterious planet's surface, and will risk all to ensure no other may claim the daemon blade lurking there: the one that calls itself Sha'eillarstill. With the promise of power and glory all too alluring, a Slaanesh-aligned warband of chaos space marines dubbed the Angels Arisen have entered orbit above Bachramshedannan, eager to earn the Dark Prince's favour by recovering their most precious relic. The reward will surely be nothing less than daemonhood, even if every soul in the system is the cost.
So too do the pus-ridden Bloated Crusaders warband seek the daemon blade, distancing themselves from Mortarion's spearhead into Ultramar to pursue this secretive agenda. They will claim Sha'eillarstill and bleed it dry, using its energies to develop a most virulent toxin to aid future conquests.
The Imperium of Man is not blind to these chaotic agendas, and have vowed that none other than the righteous custodians of Shield-Host Dracon will take the accursed blade and destroy it once and for all. This small, elite cadre of warriors will strike swiftly, swiping Sha'eillarstill and leaving while other forces are too distracted with each other.
Uncaring for the lost toys of warp-entities, the Immortal Phalanx of the Vahhasün Dynasty have journeyed to this nearby world to harvest deactivated necrons buried beneath the planet's surface, lost since the War in Heaven. They had not expected so many other combatants to disrupt their excavations, and have now summoned more forces to secure their ground.
Fortunately for the Kabal of the Aetherwave, no amount of disruption has managed to damage Bachramshedannan's webway gates. Prowling into the undergrowth, they wish to settle a score with the Bloated Crusaders, and make a tidy profit along the way by extracting the World Spirit's many ancient treasures.
Uncaring for rivalries and treasures, the looming bio-ships of Hive Fleet Ceto linger in the planet's orbit. Bachramshedannan's lush habitats are ripe with biomass, and if left uncontested, the tyranids threaten to devour the whole world, leaving all prizes forever lost in a sea of teeth and tendrils.
Each faction fought and bled, tearing at each other restlessly while scouring the planet for the world spirit's obscured entrance. The Angels Arisen struggled to make progress, having their landing zones caught in the first wave of Hive Fleet Ceto's invasion. Despite this, the tyranids did not devour the planet as quickly as expected - the other assembled armies took care to misdirect their efforts, and prepared significant defences.
Lancing strikes from the custodes were sudden and brutal. Teams of dreadnoughts and sagittarium would teleport into the fray, tearing up any camp or fortification that was suspected of hiding the world spirit's entrance. Some of these encounters were costly however, particularly when Captain-Commander Drakensvult had his jetbike stolen by the Kabal of the Aetherwave. The drukhari had been a consistent nuisance for all parties, but took particular care in harassing the Bloated Crusaders, who were responsible for an embarrassing defeat years prior.
The Bloated Crusaders were repelled at times as they scoured the planet for clues, but incredibly tedious to keep down. Any defence would eventually crumble, as the relentless march of Mortarion's sons pressed ever onwards, only seeming to come to a halt at the door of the Vahhasün Dynasty's territories. During the conflict, the necrons had rarely expanded beyond their established area of influence - and for good reason. Their intense excavations had uncovered the entrance first, and all forces were dedicating to its defence.
Due to the scheming of the drukhari and harassing strikes by the Angels Arisen, the other parties soon learned of this secret, with even the tyranids honing in on the sudden concentration of biomass. As the necrons fell back into the subterranean shrines, battles were fought no longer on the planet's surface, but through the ancient wraithbone passageways of the world spirit.
Dionadar's army of wraiths stood as the final hurdle. Ad-hoc alliances were formed, then immediately broken, as fighting became desperate and disorganised. Through a maze of passages, each warlord eventually arrived at the world spirit's core, where the Bonesinger stood. Old and as he was, he remained defiant in the face of the invaders, and a climactic battle began. Each warrior fought not only each other, but a room shaking with the hammering feet of wraiths, flooded with the psychic prowess of an ancient seer, and slowly becoming encircled by tyranid hordes.
As all seemed lost, Dionadar took up Sha'eillarstill and charged. Vassen, phareon of the Vahhasün Dynasty, strode forth to meet him with no fear of the relic's immaterial potency, and dealt the final decapitating blow. Vassen would be the one to unleash its power once again. Lashing onto the bonesinger's soul and hijacking his mortal form, Sha'eillarstill was freed into the material world, and the daemonic incursion of Bachramshedannan began anew. Too few in number to contain the tide, each faction made a hurried retreat back to the surface. Not even the chaotic warbands present were spared the daemon's wrath, for Bachramshedannan was Sha'eillarstill's world, and hers alone.
Her influence uncontested, and a world of spirits to feed her, Sha'eillarstill corrupted the planet to its very core. In the night sky above imperial hives, through the viewports of roving ork ships, on the star maps of necron tombs, and in the mad visions of chaos mystics, all could witness Bachramshedannan's transformation. At one time, the tiniest spec on any solar chart - now a swirling mass of otherworldly energy, lashing out into the depths of space.