The War of The Beast
The War of the Beast is the focus of the The BeastLeast Arises series and is one of the major events in 40k history in which the Imperium got the biggest buttfuck it would ever get in between the Horus Heresy and the giant clusterfuck of the late 41st millennium.
The Imperium at the time
Believe it or not, during the years prior to the biggest WAAAGH! ever been the Imperium was living a time one might call peaceful. Yes, there were silly little wars here and there such as the first Black Crusade, the first battle of Cadia, the Battle of the Fang, multiple traitor Primarchs fucking over many worlds, not to mention all of the Daemon cults that were popping up everywhere, but if we ignore all of those 'minor' events/chaos invasions we're left with a few others which basically consisted of the Imperials wiping out some xenos races. The Astartes were dying out because the author forgot the other shit that happened, the Imperial Fists-for example- hadn't left Terra for centuries and it was said nearly none of its battle brothers had had real battle experience in any substantial terms. The Emperor's Herd had grown complacent.
The Catastrophe: slaughter at Ardamantua
For the first time since centuries the Imperial Fists left their role as praetorians of the throneworld and went on to slaughter alien scum known as the Chromes who were considered little better than non-sentient vermin.
The Fists were caught by surprise by strange gravitational hazards which started chewing up the landscape and causing their starships to fail and crash, thus they were dealt severe losses. The High Lords of Terra reluctant to lend their own aid to the Fists on what they considered a trivial matter of fighting Chrome pests, so they allowed the remaining Fists who were guarding the throneworld to deploy to Ardamantua, along with a contingent of Guard forces commanded by the Lord Commander Militant Heth.
When the counter attack was taken into effect, an ENTIRE MOON materialized in the system; the process by which it was traveling had been messing up the planet's gravity, practically destroying it, while causing navigational hazards which scattered the fleets and crushing them.
Then appeared the WAAAGH! Beast. It was said that its boyz were as strong and big as space marines, as tall as them and quite cunning to boot. Its nobs were massive. But its biggest threat was the Ork Warboss: the Beast, (who will bring much) Slaughter. He was a leader with a great deal of tactical acumen and sophistication, going as far as being to shape the ork forces into a semi-cohesive force capable of using tactics and making frigging diplomats!
Lord Commander Militant Heth died as his forces where slaughtered right after warning the Imperium of the greatest threat they had ever faced before. The Imperial Fists were literally slaughtered to a man. There remained only one survivor: Captain Koorland. The future chapter master of the Imperial Fists and then Lord Commander of the Imperium.
Galaxy Spanning threat
The WAAAGH! Beast threatened to engulf the whole galaxy: the guard was bogged down in many fronts, the space marine chapters were mostly tied down trying to fight the invading orks. The Ultramarines had 2 of their worlds invaded and had been forced to bring in successor reinforcements. Many chapters were annihilated as they struggled to fend off an enemy the likes of which they hadn't ever faced. Worst of all was the fact that the orks were DEVELOPING technology, having such things as gravitational whips and massive teleporters capable of teleporting whole armadas in the blink of an eye.
The Imperium couldn't help but lose ground as Lord High Admiral Lansung, leader of the Imperial Navy and main topdog of the High-Lords, focused more on deploying his fleets to gain favours from other high lords and protect his own interests rather than ferrying the Imperial Guard and fighting the ork fleets. The rest of the lords were about as incompetent barring one man: Drakan Vangorich, the infamous Grand Master of Assassins who would later stage The Beheading. The man was a patriot and tried his damn best to get the imperials to move their damn fat asses. The pay off took a while but it was noticeable.
Battle for Port Sanctus
After a lot of political maneuvering and tug-o-war, Lansung decided to move its fat ass into fighting the orks (mainly because one of his admirals had broken his orders and he didn't want to lose face) for the shipyards of Port Sanctus. There half of what was a segmentum's fleet fought the Orks. And it went horribly at the beginning: the ork attack moon's gravity whip was so powerful that BATTLESHIPS were torn apart by it and the moon's WAAAGH! gate brought in constant reinforcements to fend off the imperial navy as thousands of boarding ships and small cruisers sent in boarding parties to the imperial vessels, even with the void shields up and the gellar fields on. Lansung decided he couldn't take more of this shit, and for the first time in two whole novels of total incompetence he went on and got shit done: he led a suicidal assault towards the moon. Dozens of battlecruisers and battleships were lost to the Imperium but the void shields of the gargantuan construct were drained by the energy consumption and the Imperial Navy was able to obliterate the threat.
Terra under threat and the Proletarian Crusade
But all wasn't so nice. Yes, the Navy had had, at a great cost, destroyed the attack moon (and closed its WAAAGH! Gate) but the thing was that the orks had more of those buggers around. And one of them had recently popped out... on Earth. Right then the Navy assets were on Port Sanctus and the massive network of star forts was still in construction. Basically the throne world was under threat and there was nothing that could defend them. At that time Lansung's power had been almost lost as many of its political allies turned from him, a new figure taking his place: Juskina Tull. Tull represented the chartist captains and had had a "brilliant idea" (read: a suicidal, stupid idea): lead a crusade made by the people. The idea was to take the massive merchant fleet available in Sol and load it chock full with people: guardsmen and civilian alike. Literally billions where ferried into ships that ranged to massive mass conveyors to tiny shuttles in massive tide of steel. The ork defense forces shut down a great number (if not the majority of) of the vessels but the Imperials still managed to land millions of sods and hundreds of vehicles.
There they frigging zerg rushed and beat the crap of the ork forces on the surfaces... only to be crushed by mountains. The orks manipulated the surface of the moon and shifted the mountains close to a door to the moon's interior. Trapped between the stones, the imperial forces were crushed by their own bodies and the master-crafted trap the orks laid. Thus ended the last ditch effort of the Proletarian Crusade, with a single survivor: a female adeptus arbite.
Oh, and the Eldar show up in the Imperial Palace trying to warn the humans not to let the Orks distract them from Chaos, cuz, you know, chaos has to be your #1 priority even though the Imperium is being torn apart by killer Ork death stars. This cryptic crap achieves absolutely nothing other than getting the hard-line Inquisitors into a frenzy of saying "I told you so" to anyone who will listen.
The Last Wall, a Legion unified
Dorn had envisioned the chance of something BIG happening. So he had ordered his sons that, should the need arise, they reunited at Phall, where the Heresy began for the Imperial Fists. There they would unify into a single legion and fight as one again, albeit keeping their traditional heraldry. The note was sent with not much time in advance but the majority of the second founding chapters managed to be present. There Koorland revealed to the utter shock of his fellow chapter masters from the Black Templars, Crimson Fists, Excoriators and the Fists Exemplar that he was the last imperial fist alive. The truth was almost too much for them to process but in the end they decided to get over with it and banded together. Over 3000 astartes and 50 ships went into Terra and let a glorious assault to neutralize the attack moon.
The fleet was joined by the Iron Knights chapter and succeeded in destroying the WAAAGH gate at the heart of the moon and rescuing the remaining human prisoners (including the aforementioned female Arbitrator) but unfortunately leaving the moon intact. while also suffering substantial casualties, amongst which was Chapter Master Malfons of the Iron Knights, whom sacrificed himself to save Koorland from an Ork warboss the size of a Dreadnought.
The Imperium Strikes Back
After the battle of Terra and the blockade of the attack moon, the Last Wall (the new name for the Fist Legion) were praised by The Lord Guilliman (also: Lord Commander of the Imperium, also: Warmaster, also: your boss) Udin Macht Udo (a fellow more useless than Abaddon) who wanted to make a show of having five chapters bow to him and invited them to a party. He also wanted to keep the loss of the Imperial Fists chapter a secret for public relations and ordered the successors to play dress up and give Koorland an honour guard. Udo would also show himself "gracious" enough to ratify Koorlands's position as Chapter Master, since according to rules lawyers, the successors aren't allowed to do that.
The Fists were explicitly ordered NOT to destroy the attack moon that remained in orbit, because the Adeptus Mechanicus wanted the Ork technology for themselves. (oh the irony) The Fists were also disgusted by the constant power playing between the high lords and were happy enough to leave. Suffice to say, the space marines consider them all dicks.
So that thing about Chaos? Orks are way worse
Despite the warnings from the Eldar, Chaos, or more specifically: some Iron Warriors were also getting their asses kicked by Orks. While also being hunted by some Black Templars with a grudge. Realizing that the fight won't end well, they decide to join forces temporarily, but still can't overcome the Orks. When a relief force of Fists Exemplar show up to pull them out. However, the Orks use their gravity based weapons to SPAWN A FREAKIN' BLACK HOLE, which destroys the star system, the Imperial fleet escapes by scattering into the warp, where one ship turns up on Prax along with the Iron Warriors.
Prax turns out to be a key world in the new "Ork Empire" and reveals that the Orks have been evolving even further than anticipated. Where previously they were considered to be little but savages who could hammer bits of technology together and make it work, they now have their own society and specialists, "Bloody Axes" and "Leering Moons" and so forth. They actively conquer and enslave other races; taking humans, feeding them steroids and farming them like cattle, while some "lucky ones" get their teeth punched out, shaved bald, branded like slaves and get pushed into battle as auxilia cannon fodder.
The combined Fists and Iron Warriors then destroy the planet rather than letting the Orks keep it.
Back on Terra, Vangorich manoeuvres Koorland into taking the top job of Lord Guilliman from Udo, who seems to think its business as usual, despite the Ork threat still signalling imminent doom. The Imperial Fists strut into the Senatorium and denounce him, and Udo orders their arrest, not noticing that everyone bar the Ecclesiarch and the Fabricator General have switched sides and actually wants someone competent to take command.
The new regime
With Koorland in charge, Vangorich brought him evidence of the Mechanicum withholding vital information to the war effort because of their fanatical need to hoard knowledge (and abscond with any new Ork tech). So Koorland's first act was to land troops a military force on Mars and bluff the Mechanicum into stepping into line. After a brief "incident" which was totally not a micro civil war the Fabricator General finally started singing the Imperial national anthem and revealed that the Orks were coming from Ullanor, which had sunk into ancient legend because if it's significance to the Great Crusade.
Deciding the Imperium needed it's own legend, they went looking for Vulkan, since the Inquisition conveniently knew where he was (and had been for a thousand years). Gathering a fleet, they went to the backwater volcanic planet and found Vulkan singlehandedly defending it against MILLIONS of Orks. Their arrival turned the tide, eliminated the attack moon being constructed in orbit. Providing the first major victory of the War. Between then and the end of the war the Deathwatch was born as a means of fighting the Ork WAAAGH! with a chance of success.