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. (Or as hinted at in "The Beast Arises' exercised his office's duty to police the High lords of Terra with commendable if excessive zeal by executing all the cretinously self-absorbed, politicly corrupt wastes of carbon in one go)

Coup

It is still unknown why Drakan rebelled: if it was for the glory of Chaos, his own hubris, he was fed up with the corrupt Adeptus Terra or just did it for the giggles. The beast arise series in which Drakan is main character implies he's got fead up at the high lords being selfish pricks.

In any case, the High Lords were all slain to a man by him. If the Grand Master himself did this is not certain, yet it is a safe bet that one who holds the title of "Grand Master of Assassins" can kick some serious ass. Vangorich retreated to the Assassinorum Temple on Terra, where he was soon visited by a chapter-sized strike force consisting of Imperial Fists, Halo Bretheren, and Sable Swords. Upon making landfall their leaders immediately took bullets from Vindicare operatives to the noggins, and the survivors were soon staring into the crazy eyes of one hundred Eversor assassins. Only one Space Marine made it into the Temple's inner chambers and gave Vangorich a skull full of Bolter.

The recent "The Beast Arises" series places Vangorich in a new light. Before the massed invasion led by the Beast, the Master of Assassins was growing increasingly frustrated by the pointless bickering and armchair politicking of the Council of Terra, to the extent of damaging the Imperium as a whole and ignoring pending threats. Whether bumping off the council in order to get better results was the intention is still unknown.

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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. Or, since the force was combined forces, they didn't have their entire chapter present. This is the most likely explanation.