The War of The Beast
The War of the Beast is the focus of the: The Beast 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 millenium.
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 wars here and there quite often but they tended to happen most often on the borders and basically consisted on the Imperials wiping out the xenos races. The Astartes were dying out because they lacked a purpose, the Imperial Fist-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 substancial 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 crome...
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, 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! The fists were caught by surprise and where dealt severe loses. When the counter attack was taken into effect the attack moon which had materialized recently messed up badly with the system's gravity, scattering the fleets and then crushing them. Lord Commander Militant Heth in person died as his forces where slaughtered right after warning the Imperium of the greatest threat they had ever faced before. The imperial fists where slaughtered to the proverbial 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 anhilated 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 in keeping alive those fleets loyal to him 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 whom staged 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 manneuvering 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 ther Orks. And it when 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 stupidity): led a crusade made by the people. The idea was take the massive merchant fleet avalaivable 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 mastercrafted trap the orks laid. Thus ended the last ditch effort of the Proletarian Crusade, with a single survivor: a female adeptus arbites.
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 forget their chapter heraldries and assume their genesis: they'd unify into a single legion and fight as one again. 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 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, succeeding while suffering substancial casualties, amongst which was Chapter Master Malfons of the Iron Knights, whom sacrificed himself to save Koorland from an Ork warboss.
The Imperium Strikes Back
After the battle of Terra and the annulation of the attack moon Koorland took control of the Imperial Forces, claiming the title of Lord Commander from Udin Macht Udo, thus began the counterattack of the Imperium and in the meantime 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.