Ursulia was a Daemon World and a former Exodite Maiden World of the Eldar that had been placed under the protection of the Craftworld Biel-Tan, to which it was connected through the winding corridors of the Labyrinth Dimension of the Webway. This connection was exploited by the daemonic minions of Slaanesh and Khorne, who sought to use the planet to launch an invasion of Biel-Tan in an attempt to prevent Yvraine and her Ynarri from returning home to that Craftworld and beginning their quest to fully awaken Ynnead, the Eldar God of the Dead. In the closing days of the 41st Millennium, as Abaddon the Despoiler's 13th Black Crusade gathered its forces in earnest, the growing tumult in the Warp lead to a growing surge of Warp Storms throughout the galaxy, but particularly in the Ultima Segmentum. Ursulia was consumed by one of these Warp rifts, Warp Storm Balamet, which allowed the daemons of Khorne and Slaanesh, led by the Bloodthirster Skarbrand and the Daemonette known as the Masque of Slaanesh, to rally the forces of their respective Chaos Gods there. Ursulia was used as the base for the invasion of Biel-Tan in 999.M41, and its Webway portal became the unexpected point of egress for the servants of the Dark Gods. The portal was eventually sealed by an assault from Biel-Tan aided by Yvraine and her Ynarri allies.
The Eldar of Craftworld Biel-Tan had watched over the hidden world of Ursulia for millennia, and in doing so had repelled Ork invasions, Hrud infestations, Imperial conquests and Dark Eldar raids. Against the raw power and sudden onset of a Chaos tempest, however, there was little they could do. Warp Storm Balamet had ﬂared into baleful existence so swiftly that even the Eldar could not counter it.
The Masque of Slaanesh was poised to use Ursulia to achieve the unthinkable and unleash a daemonic invasion of the most militant of all the Eldar Craftworlds. Though it had cost her much to attain it, she had masterminded a full-scale daemonic invasion of Ursulia. Her intent was not to conquer the planet, but to use it as a staging post. Should she muster force enough to break through the Webway portal present on the world known as the Obsidian Gate, she would reach Biel-Tan before Yvraine, not only claiming a rich bounty of Eldar souls but also capturing or devouring the single greatest threat to Slaanesh's continued existence. The daemon herald had taken great pains to arrange the conquest to come, and ensure that it had a semblance of focus -- no mean feat, considering the rival forces involved. The Masque had marshalled not only her own great promenade of excess -- a gathering of Daemonettes, Seekers, charioteers and half-mortal Hellﬂayers -- but also seduced a grand battalion of Khornate daemons into fighting for the same cause.
The rivalries between the Chaos Gods had raged across reality and the Warp for time immemorial. Though the brothers in darkness were each locked in their Great Game, and though they sought the same destructive ends more often than not, they were such bitter rivals that they held an open contempt for each other. This ire often boiled over into outright war. Slaanesh, the Master of Excess, was considered a self-indulgent, preening impostor by the Blood God Khorne. Conversely, Slaanesh saw the Blood God as an unimaginative boor with all the grace of a starving hound. Their daemon minions harboured much the same attitudes, for in essence a daemon is but a psychic fragment of its Chaos God patron made manifest.
The Masque of Slaanesh was nothing if not persuasive, however, and her repertoire went far beyond the pleasures of the ﬂesh. She knew well how to exploit the compulsions of others, for she was obsession given form. The strongest souls were often the easiest to fool -- hubris and overconfdence was the downfall of champions and wise men alike. The daemon lords of Khorne were prideful indeed. It was that ﬂaw that the Masque sought to play upon, thereby binding them to her cause.
With the powers of Chaos ascendant and Warp tempests raging across the breadth of Mankind's realm, daemons found moving from the Warp to the storm-wracked domains of realspace easier than ever -- especially for one as adroit as the Masque. Still, there was no way she had the strength to break open the Runes of Warding that sealed the portal to Biel-Tan. She knew of but one daemon strong enough to break the arcane defences -- Skarbrand the Exiled One, the most terrible Bloodthirster of them all. Even then his power might not suffce.
Skarbrand was a Greater Daemon whose arrogance was so immense that he sought to slay his own parent deity in single combat. He was hurled across reality as a result, broken in body and mind. All that was left of Skarbrand was rage, raw and all-consuming. Seeing in the infamous daemon an instrument of pure brute force, the Masque had sought the Bloodthirster out, dancing her way through the Realm of Chaos to speak to him face to disfgured face.
At first, Skarbrand sought to cut the Masque to pieces with his twin axes, Slaughter and Carnage. However, the Daemonette swayed and dodged from the Bloodthirster's blows with such sublime passivity that Skarbrand stopped viewing her as a martial opponent and instead saw her as more of an inconvenience, just as a rampaging stallion might see a gadﬂy upon its ﬂank. When he had all but lost interest, the Masque told her foe of her own exile, for she had been banished by her god just as Skarbrand had been banished by his. This won the raging daemon's ear, for a time.
She spoke to him of a great wager, a contest between the daemon hosts of Slaanesh and those of Khorne. The competition would be held upon the world of Ursulia -- whosoever claimed the most Eldar lives in the name of their patron before nightfall would be proved the most powerful in the service of their respective gods.
The daemon herald's words were expertly delivered. Her beguilements were clever enough to stoke Skarbrand's eternal rage, but not to trigger a killing spree -- not yet, at least. The Greater Daemon spat, snarled and roared with contempt, for the disciples of Khorne do not idly ignore a challenge to their strength. With her greater plans set in motion, the Masque smiled from ear to ear, waltzing away to amass her followers even as the mighty Bloodthirster stomped off on his own warpath.
Within a solar week of that incongruous pact, the daemon hosts of the Masque and Skarbrand trod the peaty loam of Ursulia's twisted forests. Their Warp-born followers numbered in the hundreds of thousands, for word of the wager had brought a great many daemonic champions together, each determined to outclass their god's rivals with impressive acts of slaughter.
As the daemons burst from within the eye of Ursulia's fiercest psychic storm, the invasion had begun in earnest. The Exodites defending their world had used every weapon, trick and trap at their disposal, unleashing hordes of roaring megasaurs and mounting mass cavalry charges that saw whole households of Dragon Knights charge into echelons of daemonic foot soldiers. Theirs was a noble act of defance, but ultimately it was doomed. The invading host outnumbered them twice over, and with the Masque and Skarbrand at the fore, the Exodite defenders were overwhelmed in a matter of solar days. Ursulia became a full-fledged Daemon World.
Once, the planet Ursulia was a spectacular utopia of lush vegetation and glittering waters. Ursulia, named after a famously beautiful Eldar maiden of myth, was a small but verdant world, legendary in Eldar society for its majestic thornwoods and towering arbor cities. It had been fashioned as a true paradise by the ancient Aeldari, but it had been twisted beyond recognition. Since the breaking of new Warp Storms across the coreward sectors of the Ultima Segmentum in the last solar months of the 41st Millennium, it has been transmuted into a nightmarish vision. Its forests have become wildernesses of sentient plant life dripping with mindpoison, jagged fungi that belch hallucinogenic gas, and mutant saurian beasts covered in eyes and mouths from snout to tail. Psychic storms still ravage the ruined Eldar settlements of the planet, bringing with them sudden squalls of ectoplasmic rain, winds that scream with banshee rage, and crackling empyric lightning. These phenomena leave utter devastation in their wake from which there can be no recovery. They taint not only the planet's landscape, but its spirit as well.
To descend through the silver cloud banks of Ursulia's skies was to feel a great sadness of the soul. Roiling Warp Storms had lashed its surface in the last few Terran months of 999.M41, appearing from nowhere like a seismic eruption upon an unseen fault line. Ursulia's glorious waterfalls had been turned to swathes of crimson glass, and its rolling dales reduced to skull-strewn wastelands. Amongst the planet's valleys was a moss-strewn henge known as the Obsidian Gate. This former Webway route was permanently closed many thousands of Terran years ago as a precautionary measure against invasion, for it led straight to Biel-Tan. The decision to seal it had since been vindicated a dozen times over, for gentle Ursulia had known many wars over the millennia. Yet it was theoretically possible that the route could be re-opened by arcane force -- like that brought to bear by the servants of the Dark Gods.
- The Gathering Storm - Book Two - Fracture of Biel-Tan (7th Edition), pg. 22