The 13th Black Crusade

"Since the time of The Fall, our race has been haunted by, what we, in our reckless pursuit of hedonistic indulgence, gave birth to. Though our dreams once overturned worlds and quenced suns, we are now but fitful shadows clinging to the edge of existence. All the stars in the sky cannot blot out the hateful glare of the Red Moon's Eye. The birthing place of The Great Enemy pulses with all the malice of a daemon that is dreaming, casting its shadow over all we have ever done and all we ever shall. Every twisted strand of Fate and casting of the Runes leads me to this time, to this place, and it is clear that the final battle awaits me at the ancient Crone Worlds. A conflict the likes of which has not been seen since the Mon-Keigh warred among themselves, and the corpse of a seer fell to his traitorous son, is coming and all my steps lead towards it, no matter that I walk other paths. I see the stars stained red with the blood of the Mon-Keigh and, though their wars do not concern me and I would gladly let them destroy one another, I know that to avoid this fight is to condemn my race to inevitable doom. And though all I see is darkness, I know that I will not flinch from my destiny"

– Eldrad, musing on his (now-Retconned) doom.

The 13th Black Crusade
Date	999.M41 - Current
Scale	Multiple Sectors
Theatre	13th Black Crusade
Status	Inconclusive.
Belligerents
Chaos	The Imperium of Man, Eldar
Commanders and Leaders
Abaddon The Despoiler	Lord Castellan Ursarkar E. Creed, Saint Celestine, Belisarius Cawl, Necron intervention via Trazyn the Infinite
Strength
Black Legion, Traitor Legions (Alpha Legion, Death Guard, Emperor's Children, Iron Warriors, Night Lords, Thousand Sons, World Eaters, Word Bearers), Astartes Renegades (Violators, Warp Ghosts, Sons of Malice), Traitor Titan Legions (Deaths Heads, Death Stalkers, Fire Masters, Iron Skulls, Legio Vulcanum), Traitor Guard (5th Columnius, 666th Regiment of Foot, Discilian Apostates, Jenen Ironclads, Sentrek Freemenm the Traitor 9th, Ubridius Light Infantry, Volscani Cataphracts), various Fleets of Chaos (an estimated 38 different fleets).	Adeptus Astartes Chapters (Angels of Absolution, Angels of Vigilance, Angels Sanguine, Brazen Claws, Dark Angels, Death Spectres, Doom Eagles, Exorcists, Excoriators, Harbingers, Howling Griffons, Iron Hands, Iron Knights, Iron Snakes, Marines Exemplar, Night Watch, Novamarines, Relictors, Subjugators, Storm Warriors, Space Wolves, Storm Warriors, Ultramarines, White Consuls), Adeptus Sororitas Orders (Bloody Rose, Ebon Chalice, Martyred Lady, Ermine Mantle, Wounded Heart), Astra Militarum Regiments (Avelornian Gunners, Bar-El Penal Legions, Cadian Karskin, Cadian Shock, Cadian Whiteshields, Drookian Fen Guard, Finreht Highlanders, Gudrunite Rifles, Jouran Dragoons, Kellersburg Irregulars, Gnovian Gharkas, Mordant Acid Dogs, Narsine Yeomanry, Necromundan 8th, Thracian Guard, Van Demans World Redbacks, Zenonian Free Companies), Adeptus Questoris (House Krast, House Arokon), Titan Legions (Metalica, Ignatum, Gryphonicus, Astorum, Sinister), Adeptus Mechanicus (Skitarii regiments, Ordo Reductor, Ordinatii), Imperial Navy Battlefleets (Cadia, Corona, Scarus).
Losses
High. One Blackstone Fortress is lost, along with unknown number of ships of the Black Fleet.	Extremely High. The Cadian Gate is lost, and the Great Rift tears the Imperium in twain.
Outcome
*Cadia is destroyed, and the Cadian Gate is blown wide open. The Eye of Terror grows into the Great Rift, and cuts the Imperium in half. While the Imperium has managed to stabilize things on the side where Terra is, thanks to the resurrection of Roboute Guilliman, Chaos forces now have free reign over what is now known as Imperium Nihilus.

Launched in what might be considered the final hours of the 41st Millennium, the 13th Black Crusade is Abaddon the Despoiler's latest attempt at trying to bring down the Imperium -- and very nearly did so. As of the current timeline, its still ongoing, with no signs of slowing down.

Signs of the End

A Creeping Doom

In the chaos of the end of the millennium, the first signs that Abby was up to no good was not the appearance of war fleets. Oh no, instead what happened was the sudden jump of sightings of abandoned vessels getting spat out of the warp at in the sectors surrounding Cadia. Though rare, such a large instance hulks showing up at the same time wasn't unheard of either, though their convergence upon the core worlds of the systems merited enough of a response that system defense ships had to be scrambled to deal with them.

Warriors of the Adeptus Astartes were also among those called to intercept these derelicts, but what they found were disease-ridden nightmares filled with toxic filth. When news of this spread to the rest of the Navy, every wreck subsequently encountered were destroyed ASAP with torpedos and macrocannons from afar, but by then it was too late.

Outbreaks of sickness soon broke out among the ranks of the Imperial Navy, which coincided with the sudden sighting of the Chaos warship called the Plagueclaw in the Urthwart system. With more and more navy personnel in the region fallen ill, the number of intercept-capable ships dropped as well. Despite this the caravan of diseased ships continued to drop from the warp, all in direct course for important worlds. Ad-hoc groupings of ships formed to try and destroy the hulks before they reached those planets, and an unofficial fleet under Admiran Quarren assembled in Belis Corona to hunt down the Plagueclaw.

What it encountered instead was far, far, worse. In the Frenerax Dust cloud, the battle group was jumped by the infamous Terminus Est, flagship of Typhus, the Herald of Nurgle. Outgunned, the fleet still managed to fight its way free, but for some reason Typhus deigned not to pursue.

Unbelief

Despite barely surviving from the encounter with Typhus, it was not yet over for the crews of that battered fleet. Many would sicken and die from disease on the return trip to Belis Corona, but the fleet still managed to return with it ships still usable.

Compared to Belis Corona though, it was pretty bad. Many plague hulks had slipped through the line of System ships arrayed against them, and the same disease that had infected their crew spread like wildfire through the inhabited worlds of the Cadian and Agripinaa sectors. As the plagues killed millions, panic soon spread, and inter-system traffic was halted in an effort to stem the spread of infection.

As despair settled among the populations, apocalyptic cults began to appear on every benighted world, and preached that the Emperor's wrath had descended upon everyone as punishment for their wickedness. Only the faithful would be spared the Curse of Unbelief, and masses of self-flagellating devotees began to fill the streets of these worlds, attracted by both their fiery rhetoric as well as their surprising health.

And then plague zombies happened. Because why not?

Within months millions of plague zombies clawed themselves out from the makeshift graves they had been buried in among the dozens of worlds in the Belis Corona and Agripinaa sectors, and threw themselves on their still-living relatives. The Astra Militarum are pushed to the breaking point trying to contain both the zombie outbreak and the mobs of zealots running amok, while the Imperial Navy, already crippled by the epidemic burning itself though its crews, was in no condition to stop what was to follow.

A massive Chaos Fleet, with the Terminus Est at its head, surged into Imperial Space, and there was nothing in its way to stop it.

Meta

The 13th Black Crusade is literally where the "clock" stopped in the Warhammer 40,000 for more than two decades and several editions, before a new management decided to move things forward finally for 8th Edition.

There have been two differing outcomes for this Black Crusade as far as lore goes -- one from the Eye of Terror Campaign, and the one for Gathering Storm, the latter of which became the new status quo for the setting.

Humorously, the latter actually still treats many of the beats of the Eye of Terror campaign as canon, just as long as it doesn't contradict things outright.

The Eye of Terror Campaign

Back in Third Edition, GW ran the "Eye of Terror" worldwide campaign to determine the outcome of the war. Going by the numbers of the battles sent in (admittedly not a tamper-proof measure), the forces of Chaos actually won the ground war, and the Imperium won the space war, since this was before GW had the "fuck our customers" mindset, they actually did wrap up the 13th Black Crusade...

In the 18th issue of their Battlefleet Gothic Magazine.

Yep, the major event that the whole setting stops at was wrapped up in an issue nobody bought (so a lot of people thought it never ended). Abaddon's fleets (including the Planet Killer and a Blackstone Fortress) were crippled and he and his generals were stranded on a dozen worlds. They continued to win the ground war on most planets, including Cadia, but even if they did capture those they'd be left stuck without any way of proceeding whereas the Imperium could just virus bomb the captured worlds.