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 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

Of course this didn't sit well with the Imperium at large. After all the many reformations that had been laid across the last millennium were meant to prevent exactly that (and certain guy named Goge showed they were there for a reason). Thus a strike force led by the Imperial Fists landed on the Assassinorum's headquarter to hunt down Drakan. The Officio responded in kind deploying its most lethal operatives: the landing zones were killzones filled with snipers and the disembarking Astartes were assailed by a cavalcade of roaring Eversors. Callidus and Culexus further harassed their advance.

The marines were killed to a single man who proceeded to kill Drakan with his bolt pistol (kind of a weak way to go, we pray the novel does them more justice) and thus the High Lords had been totally wiped out.

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.