"The Great Rift has torn the Emperor's realm in two. Ancient alien empires rise from the ashes of prehistory to claw at the dying stars. Predatory nightmares boil from the outer darkness while everywhere the veil of reality frays thinner and the flames of war rise higher. Day by day, war upon war, the galaxy careens ever closer to total annihilation. From amidst the growing anarchy and prophecies of a final ending rise those who would deny the last darkness and, in so doing, seize victory for themselves. Greatest amongst these is Abaddon the Despoiler.""The galaxy writhes like a wounded beast. Tormented by the infected sores of countless warp storms, it laments in agony. While the forces of the Imperium Sanctus fight tooth and nail to regain control, the worlds of the Imperium Nihilus endure besiegement by nightmares uncounted. The forces of Chaos attack on every front, from the Nachmund Gauntlet to the Charadon Sector. Alien races battle furiously for survival, or rampage across the stars in orgies of conquest and destruction. Yet no one faction can truly gain mastery. Every victory and defeat only sees the inferno rage hotter. Abaddon the Despoiler has not labored for thousands of years to see his Long War end in mutual annihilation. His hands move unseen through the shadows on secret and diabolical missions. His legions of sorcerers and cryptomagi pore over every shred of arcane lore and prophetic utterance ever gathered, hunting for some esoteric advantage that will allow the Warmaster to seize decisive control of the conflict. Abaddon desires a secret weapon to unleash upon the Imperium of Man, and he will tear apart time and space if he must to secure it.""The galaxy burns. From the far-flung worlds of the Uhulis Sector to the nightmarish Ghoul Stars, from lonely Repentance to the bloodied systems of Scarus, war rages upon every world. Since the emergence of the Great Rift, countless systems and species and been plunged into desperate conflict like never before. The hour is as dark as it has ever been.
A nightmarish new entity steps from the depths of the warp to join the battle for realspace, himself but the first herald of shocking revelations and galaxy-shaking cataclysms to come. Meanwhile, old hatreds rule and dark legends are unleashed once more upon the stars, to the potential ruin of all.
At the heart of the raging storm stands Abaddon the Despoiler, master of the innumerable hosts of Chaos. Here begins his quest for a weapon of ultimate power that will finally and decisively turn the tides of the Long War in his favor. Dread new allies gather to the Despoiler's banner in this latest onslaught against his galaxy of foes, while grim-faced lords of battle step forth to defy him. And above them all -- around them all, booming like unholy thunder through the warp -- the laughter of the thirsting gods echoes ever on..."Abaddon is basically king of the entire Imperium Nihilus now, minus a few holdouts here and there, essentially meaning he rules half the galaxy now. More territory than Horus ever conquered or held and done without selling his soul to the Chaos Gods, as he likes to boast. He officially goes by the title "Warmaster of the Imperium Nihilus".
Abaddon has discovered an ancient prophecy and is determined to see it realized, as he believes it could lead to him achieving absolute victory.
Abaddon knows about the Silent King and the Pariah Nexus, "Another image showed him a smothering shroud cast across a mass of screaming faces and clawing hands, dragging them down into a still black lake. That image came again and again, proliferating about the edges of the shimmering starfield until Abaddon swept it away. He was well aware of the Necrons and their king unspeaking, and had yet to decide how to respond to the threat they posed." That speaks volumes, given how much Abaddon likes to plan ahead.
Vashtorr and Abaddon enter into an alliance; they need to go on a fetch quest to gather artifacts and relics from various epochs and civilizations with unique Warp signatures so Vashtorr can create a mystical key to unlock something, something that will supposedly allow Vashtorr to ascend to true godhood and give Abbadon a weapon to deliver him true and final victory in the Long War. Neither can acquire it alone, but together they can realize the prophecy and alter the fate of the galaxy forever. They don’t need to claim every relic to make the key, just enough, and Vashtorr’s demigod powers can do the rest. The nature of what they're trying to unlock isn't known to us, but upon being told what it was in hours-long secret negotiations with Vashtorr, Abby's laughter echoed throughout the Vengeful Spirit.
Vashtorr is unique in that he doesn’t lie, he’s the only honest daemon in the Warp. He lays out his intentions all upfront, nor does he deceive or cheat or use trickery in his contracts or bargains. If you make a deal with Vashtorr and get screwed because of it, you literally only have yourself to blame, no excuses; you walked into your own damnation of your own free will, and Vashtorr finds that to be more entertaining. He’s basically the demigod of science and invention and technological progress, though shorn of all compassion, empathy, pity, or mercy, while also being the arms dealer for the Big 4.
Abaddon arrives at the fortress-sanctuary of the Iron Angels Space Marine chapter to get the first relic and utterly wipes them out. They're gone-gone.
Vashtorr and Abby build the Arks of Omen, and assign them to various renegade and Chaos-aligned warbands, chapters, and legions, so they can spread out their reach as far as possible and not shed more Black Legion blood than necessary, as they will be needed in the days ahead. Various warlords compete for Abby’s favor, or try to steal or destroy Arks for themselves, so there’s a lot of Chaos vs Chaos going on. Some Chaos factions see this as a chance to dethrone Abaddon, size power for themselves or their patron god.
Arks have various daemonic machinery inside them, including something called an Obelyskane, which is a Chaos version of the Astronomican.
The Balefleets go out and pursue their missions. Once they retrieve the relics they're assigned and return them, the commanders are free to keep their Arks and use them as they see fit. Abby seeds the Arks not only with Black Legion loyalists to keep an eye on things, but he also cashes in a favor from the Alpha Legion who put in agents in the event the assigned Ark commander decides not to honor their commitments.
While the attacks from the Arks across the galaxy seem random at first, various Imperium Inquisitors and Eldar farseers start to see the patterns of a wider strategy at work, even if they're still in the dark of what the ultimate objective is. The threat is so big its got Eldar of every faction and the Imperium working together against this.
One unlucky Ark and its Balefleet pops out of the Warp right in the middle of a Tyranid Hive Fleet; they get quickly swarmed and annihilated. One Ark gets taken down by Orks, one is being hunted by vengeful Necrons, one has to chase down a Squat league deep in the core, one gets stolen by Huron Blackheart when it gets too close to the Maelstrom, several get taken down by various Eldar Craftworlders/Corsairs/Kabals/Harlequins, several by the Imperium, etc. One Ark of Omen was boarded by Dante's Blood Angels, only to discover that the entire crew was already slain but some unknown party. No definitive answer is given as to who was responsible, but there’s some nuggets of speculation that have spawned some interesting theories on the 40k reddit threads.
Abaddon has to handle one fetch quest personally after the Balefleet assigned to it got destroyed by a bunch of Khorne daemons who thought today was a good day for team-killing, and after Abby deals with that bullshit on the Ark, he's further annoyed to discover that upon making it planet-side, the bulk of the Imperial forces he has to fight through to reach the artifact are Cadian survivors who just won’t shut up about avenging Cadia. He goes to town on them, and slaughters them like Vader killing Rebels in Rogue One.
Vashtorr warns Abby of an unexpected complication: that a particularly important relic they absolutely must have for the ritual on the world of Malakbael won’t be able to be retrieved by any one of their Balefleets because the entire fourth fleet of the Indominus Crusade is protecting it. Abaddon reacts to this news by just laughing and sending Angron, to which Vashtorr is like, shit, I’m going to pull up a chair and watch this, carnage like this doesn’t happen often. Angron is basically the equivalent of a tactical nuke for Chaos. When defenses are too tough to crack, call in Angron.
Part of the prophecy:
"In my dreams the sky is burning. The stars blaze bright enough to drown out the darkness around. My heart leaps to see their lights, and I weep to feel such hope.
Then the stars start to die.
Faltering.
Guttering.
One by one they burn away and crumble to ash that falls thick as snow. It lands upon my upturned face. It mingles with my tears. My hope has turned to horror, and as I drag my fingers down my cheeks, they trail smears of darkness that scald my skin.
I sink to my knees amidst the ashes. I know that this is the end.
Of me.
Of everything.
Yet in that moment I am seized by sudden knowledge. Revelation. I can stop this! I tear my gaze from the dying heavens and I dig, terrified that I am already too late. Something lies under the ashes. Something has been hidden deep, something with the power to stop the stars from burning out.
My clawing fingers uncover a pattern in the ashes. I see the suggestion of furnaces and screaming mouths, of chains, of hammers. At their heart stands a figure with eyes of burning amber. It turns.
Terror grips me and I dig.
There's another pattern sitting below the first. A fortress, vast and mighty, but gouged by the beasts raging around its walls. Again the cinders dance before my eyes, racing across the ashen bastion like cracks. Just before the image shatters those glowing lines of embers form the shape of a crimson giant.
Above the stars die, and die, and die.
I dig.
I see an ashen forest grow, and wither, and grow again.
I see an ancient knight kneel upon his field of victory and weep amber tears.
I see a circle, within a circle, within an circle, inward and inward until at their heart... nothing.
The light grows faint and still I dig. I cannot be too late.
And then at last I have it. The key is an ugly thing of brass and bone and rock. Its rusted surface is hot, burns my bleeding fingers. I cannot care. I must end this.
But a key must have a lock.
Even as the dream begins to fade I feel my gaze dragged back to the hollow skies above. I don't want to look, but I must.
So few stars remain, and as they swirl together they become a single searing point so bright it hurts my eyes. I know that I have found the keyhole. Yet even as my shaking hand raises the key on high I am gripped by a different fear.
What has been imprisoned?
What will I unleash?
My doubts redouble and I feel suddenly as though I am being used. But now the dream has me and I cannot resist.
The key slides into a lock that blazes with the light of dying stars.
It turns, and flames engulf me.
I wake screaming, nails torn from swollen fingers, bloody tears streaking my face. Sometimes, if I am lucky, it is days before I dream again.
But I always dream again." 
