Betrayal is second nature to the scions of Chaos. After all, the Traitor Legions are defined by their infidelity. Thus, at Abaddon's command, the Black Legion launched a sudden orbital drop directly into the path of the Dark Angels.
Simultaneously, the ambitious Chaos Space Marine warbands turned their predatory gaze toward the forces of the Dark Mechanicum. Seeing an opportunity for plunder, these destitute traitors immediately began seizing assets and looting the technological horrors of the Dark Magi.
"The Despoiler's curs!"
When the Dark Angels beheld the sudden emergence of the black-clad fleet and the descent of their drop pods, a fire of ancient hatred ignited within them.
The Risen standing beside Lion El'Jonson immediately turned to their gene-father. "My Lord, grant us the honor of purging these traitors for you!"
Though rehabilitated and brought back into the Imperial fold by the Lion's grace, these Risen remained pariahs to the Unforgiven. For ten thousand years, the Dark Angels had hunted the Fallen, expunging the secret's witnesses across the stars. Now, with the sudden return of these former renegades, even with the Lion's personal absolution, the rift between brothers remained unhealed.
As the Lion had famously asked: "Would my sons refuse my command?"
In truth, they nearly did. In a very short time, the Risen had learned the true meaning of malice from their own kinsmen. Consequently, they were desperate for any chance to prove their loyalty.
However, Lion El'Jonson shook his head. Looking upon the Black Legion with a complex expression, he spoke: "No. We withdraw. There is no need for my sons' blood to be spilled in this senseless mire. Let these foul wretches tear one another apart."
Despite his burning martial pride, the Lion made the strategic choice. The Dark Angels had already suffered heavy casualties during their assault on the Idolatros System and the iron fortresses of Wyrmwood, compounded by the subsequent onslaught of Angron and his World Eaters. The Chapter was bled white; they could not endure another war of attrition.
"As... as you command, Father."
The Dark Angels committed to the retreat, signaling their fleet for extraction.
I must warn that fool Guilliman, the Lion thought grimly. A new enemy has beset the Imperium.
Possessing the near-infinite knowledge of the First Primarch, the Lion recognized these upright, long-tailed, fur-clad xenos. They bore a disturbing resemblance to an ancient creature of Holy Terra: the rat. Yet, he could not fathom how vermin had evolved into such a xenos threat.
"Squeek-die! Die-die!"
Fueled by injections of warp-hallucinogens, Skaven Slave-Rats clutched warp-bombs and charged the Dark Angels' lines in a suicidal frenzy. Those picked off by bolter fire erupted into miniature green mushroom clouds of baleful energy.
From the rear, Warplock Jezzails maintained a relentless cadence of fire. Though the Astartes could dodge the shots with their superhuman reflexes, a single hit was almost invariably fatal. Even a glancing wound to a limb allowed the warpstone bullets to instantly corrupt and liquefy the transhuman flesh of the invulnerable Astartes.
Many Battle-Brothers watched in horror as their comrades transformed into piles of jagged crystal or mounds of writhing, skinless vermin. Such an ignominious end was a nightmare even for the Adeptus Astartes.
The Lion, the paragon of knighthood, personally led the Risen in a rearguard action. Facing dozens of Skaven Doom-Flayers, whirring engines of bladed death, the Lion raised the Emperor's Shield and met the charge head-on.
With a thunderous crash, the poison-coated blades and the crude, malevolent machinery of the Doom-Flayers shattered like glass. Before the Skaven pilots could scramble from the wreckage, the Risen crushed their skulls beneath their ceramite boots.
"Everyone, fall back!"
Sensing that this titan of a man was far beyond their ability to slay, the Skaven began to hesitate. The Dark Angels took advantage of the lull, extracting from the battlefield at a bitter cost.
Lucius offered no objection. Expecting Clan Skryre to detain the First Primarch was a fool's errand. His attention was focused elsewhere, deep within the Warp.
As the Dark Mechanicum forces in realspace were dismantled by the combined treachery of the Black Legion and the Skaven, Vashtorr the Arkifane's reinforcement pool dwindled. The Daemonic Demigod of the Forge surged with desperate fury.
Beset by dozens of Chaos Space Marines, Vashtorr raised a hand. The metallic scrap littering the ground began to undulate and writhe as if alive. In an instant, the iron refuse transmutated into a forest of steel blades and spikes, impaling the mortals who dared strike at him.
Be'lakor's disciples were cut down in an eye-blink, but then, a pair of massive obsidian wings beat the air. The Dark Master, Be'lakor, descended before Vashtorr with a sneering grin.
"Oh... poor, pitiable Vashtorr. Where is your grand design now? Did you truly believe you could ascend to true godhood before me?"
Be'lakor's mockery was biting. Jealousy consumed the First Prince; the thought of a demigod of comparable power achieving divinity first drove his spiteful mind to the brink of madness. He had sought out the Great Horned Rat specifically to ensure Vashtorr's failure.
"The Forge of Souls is mine, it can only be mine, you wretched thing."
Vashtorr hissed with a mixture of rage and contempt. "First Prince, you dolt! Do you still not grasp the laws of the Warp? You may occupy the Forge, but you can never possess the faith of mortals. Does your primitive mind truly think it can command the devotion of the Machine?"
"Heh, that is no concern of yours!" Be'lakor roared, swinging the Blade of Shadows at Vashtorr.
While Vashtorr possessed a superior intellect, he was no match for Be'lakor in a primal duel of martial strength. The First Prince's onslaught was relentless. Before long, Vashtorr's mechanical form was hewn into scrap.
With a final, decisive stroke, Be'lakor decapitated the Arkifane. As the mechanical eyes flickered with a hateful red light, Vashtorr's physical shell dissolved into a cloud of warp energy and vanished.
With Vashtorr banished, the daemonic entities bound within the Forge began to unravel. The once-impregnable iron defenses turned into useless obstacles in an instant.
Suddenly, Be'lakor's Disciples and the Rat Ogres they had been fighting alongside stood face-to-face, eye-to-eye, weapons bared and ready to turn on each other.
However, Be'lakor stayed his followers' hands. He looked toward the towering, white-furred Fanatic-Plague Lord, Kritislik, and spoke:
"Peace. Be'lakor honors his pacts. Tell the Great Horned Rat that, as agreed, I shall share the Forge of Souls with Him."