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
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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.

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.