The Beheading

The Beheading was an event in 546.M32 where Drakan Vangorich, the Grand Master of the Officio Assassinorum rebelled against the High Lords of Terra.

Coup

In the mid-32nd millennium a great calamity befell the Imperium: the WAAAGH! Beast. The most vicious rampage made EVER by orkind drove the Imperium to its knees as dozens of marine chapters were annihilated and thousands of worlds were put to the torch. Terra itself was in a state of constant threat during a good chunk of the war.

The high-lords reaction varied from total incompetence to out-right hindering (special note goes to the Ecclesiarch who wanted to WORSHIP the Orks and abandon the cult of the Emperor) the war effort and thus costing more worlds and billions if not trillions of lives. Except one (later two): Drakan Vangorich himself, a guy who was a bonafide patriot. He and chapter Master Koorland made great efforts and maneuvers to get the filthy and complacent asses of the high-lords to work on the threat at hand. It suffices to say the two of them may be amongst the greatest heroes of the Imperium, for without them mankind would have been annihilated or reduced to slavery a la the Age of Strife.

Once the war was over, Drakan realized that, should there be another threat of a similar caliber, mankind would be wiped out due to the incompetence of the High Lords. This was especially evident when, as The Beast sent his diplomats asking for their surrender, the High Lords were more concerned about their own political power than saving mankind. For the sake of the Imperium, they needed to die (and die they did).

The final straw was when Vangorich found out that the Fabricator General had teleported Ullanor away (instead of Exterminatusing it), he promptly flipped his shit and killed / replaced every other member of the High Lords within 5 days. Vangorich himself was left top dog of the High Lords and had the rest of the council replaced with impostors or pupputs making him ruler of the Imperium in all but name but grew increasingly paranoid about the Imperium ever backsliding back into the state it had been before the Beast invaded. He effectively ruled the Imperium for the next hundred years but his reign was marked with massacres, vicious reprisal, harsh punishment and increasingly totalitarian control. The Imperium remained strong and united (recovering well enough to sustain the Fourth Founding) but Terra finally grew sick of living in constant fear of their single ruler. The Astrates and Inquisition, agreeing on a unified goal for a change, decided that it was time to relieve him of office. Every other department in the Imperium stepped aside to let them do it.

Response

A strike force of 400 marines led by the Imperial Fists and supported by elements of newly created chapters (The Halo Brethen and Sable Swords specifically) landed on the Imperial palace to hunt down Drakan and sieze control of the Senatorum. The Officio responded in kind deploying its most lethal operatives: the landing zones became killzones filled with snipers and the disembarking Astartes were assailed by hundreds of assassins from various temples. Callidus and Culexus harassed their advance, while vidicares took shots at them from every angle (probably venenum and vanus operatives probably worked from behind the scenes to hinder their advance) but strangely no Eversors assassins were present. Despite losing half there numbers in the process the Astartes managed to carve their way through the assassins and clear out the Senatorum for the most part. Thane then confronted Inquisitorial Representative Wienend who revealed Vangorich had fled to the Eversor temple at the North pole. Both had a row of words with regretting the situation before Thane departed, Wienand staying behind to die by a bomb trap Vangorich had placed in the Senatorum (she was aware of it but felt it appropriate to go out with a bang given the circumstances).

The remains of the strike force assaulted the temple and confronted Vangorich who had some final words with Thane with both accusing the other of having some responsibility for the current situation (to be fair running the Imperium would eventually drive anyone crazy, and Thane really should have stayed behind to help get things in order instead of crusading). Regardless Vangorich sprung his trap by flooding the room with smoke and unleashing one hundred Eversors on the Astartes. The two forces cut each other down till only three remained; Chapter Master Thane of the Imperial Fists, Vangorich, and a single Eversor (Beast Krule, who had been forcibly converted after betraying Vangorich earlier). Vangorich ordered Krule to cut down Thane but the Assassin paused, giving Thane time to get a bead on Vangorich. Oddly Vangorich's last words were asking if Thane would like to know how Konrad Curze died (possibly drawing a paralel between their intended roles in the Imperium) but Thane was having none of it, killing him before he could answer. Many reformations were laid across the next few millennium to prevent one person gaining complete control of the imperium (and certain guy named Goge showed they were there for a reason) and causing such trouble, such as abolishing the position of Lord commander of the Imperium.

Aftermath

Because of the lack of leadership the Imperium fell into a state of anarchy. For a century long there was no central leadership and no successors for the High Lords. Eventually Agnathio, the then-Chapter Master of the Ultramarines gathered up no less than fifty of his fellow Chapter Masters, went to Terra and they proceeded with cock-slapping everyone in the Senatorum until once again twelve High Lords were chosen.

Clash of the Canon

One of the Chapters who fought against the Assassins was the Sable Swords. The Chapter that is currently known as the Sable Swords was formed from the remnants of the Astral Knights, nearly wiped out during their siege of The World Engine… which was in 926.M41. 9380 years later. It is possible that they have been refounded after their destruction (like with the Minotaurs), but this is not being hinted in what is known about them like with the Minotaurs. Or it could have been another fuckup, who knows.

The Offical Cannon does however list the Sabre Swords as Losing over 800 Marines in the fighting. So they were most likely Massacred and re-built.