"Fire sputters...The shame of our deaths and our heresies is done. They are behind us, like wretched phantoms. This is a new age, a strong age, an Age of Imperium. Despite our losses, despite the fallen sons, despite the eternal silence of the Emperor, now watching over us in spirit instead of in person, we will endure. There will be no more war on such a perilous scale. There will be an end to wanton destruction. Yes, foes will come and enemies will arise. Our security will be threatened, but we will be ready, our mighty fists raised. There will be no great war to challenge us now. We will not be brought to the brink like that again..."
The Beheading was a coup d'etat against the Senatorum Imperialis instigated by Drakan Vangorich, the Grand Master of Assassins of the Officio Assassinorum and newly-selected Lord Protector of the Imperium. He had the entirety of the High Lords of Terra assassinated by his agents in 546.M32 so that he could take full political control of the Imperial government.
He then installed puppet High Lords and oversaw the rebuilding and reconstruction efforts of the Imperium in the wake of the War of the Beast for nearly a standard century. He became corrupted by this ultimate power, and degenerated into a mad tyrant.
Following the disastrous events of the War of the Beast -- a campaign against the largest Ork WAAAGH! seen since the time of the Great Crusade, led by the mysterious greenskin warlord known only as The Beast, the Imperium had been left a shattered and ruined interstellar empire.
Following the successful eradication of the massive greenskin invasion force and the defeat of The Beast, the Lord Commander of the Imperium Maximus Thane, the former Chapter Master of the Fists Exemplar, and now the newly appointed commander of the reconstituted Imperial Fists, decreed that the Imperium would raise as many new Chapters of Astartes as it had gene-seed in the great vaults of Terra to sustain.
Gene-seed tithing was doubled for the next standard century to replenish the vaults. When it was done, there would be hundreds of Chapters to ring the Imperium in adamantium and ceramite instead of only dozens.
The lord commander also decreed that the Imperial Fists would no longer garrison the Throneworld of Terra, as they would become a Crusading Chapter instead to bring their strength and Thane's authority as lord commander of the Imperium to the beleaguered worlds of the Imperium, to free them from alien tyranny, and speed their reconstruction.
Knowing that his labours would be long, and that he might never see the Throneworld again, Thane invested the Grand Master of Assassins Drakan Vangorich as the "Lord Protector" of the Imperium, to be his personal representative and act as his voice and his hand, to be heeded and treated with as if Thane were present himself.
To allay the High Lords' misgivings, he reminded them that the threat of his return would be enough to ensure Vangorich's good behaviour.
Following the departure of the Imperial Fists from Terra, Vangorich was left to oversee the reconstruction efforts of the Imperium and to act as Thane's voice in the council of the Senatorum Imperialis. But over the course of the War of the Beast, Vangorich had grown contemptuous of the political machinations and squabbling of the fractious High Lords during the conflict.
But he remained committed, at first, to do his best to work with them in the rebuilding of the shattered Imperium. However, Thane had warned Vangorich of Fabricator-General Kubik and his self-interest. Eventually Vangorich discovered the full extent of the Fabricator-General's plans.
The head of the Adeptus Mechanicus had utilised matter displacement technologies plundered from the Greenskins' Attack Moons to secretly teleport the Ork capital city of Ullanor, Gorkogrod, aboard the immense Adeptus Mechanicus vessel Ark Majesty instead of carrying out an Exterminatus on Ullanor as Chapter Master Thane had demanded. Kubik was intent on plundering the Greenskins for every scrap of their advanced technology. Enraged by Kubik's deception, Vangorich decided to act.
The Grand Master of Assassins understood that the Officio Assassinorum was a tool to be used to check the follies of empire. He knew what it could do and what an abuse of power it could be to use as the Emperor intended it to be used. That is why he had avoided acting, until now.
Vangorich came to the conclusion that the squabbling rabble in the Senatorum Imperialis were irredeemable. They had done nothing for a thousand standard years but stuff their greedy faces with the wealth of the Imperium. Vangorich saw no other choice but to act, for the Imperium had become corrupt from within. The deadwood needed to be disposed of, and the new given a chance to germinate.
Striking swiftly, Vangorich ordered the death of the High Twelve, which was swiftly carried out in a single solar day. Only Wienand, the joint-Inquisitorial Representative, was spared, as Vangorich harboured romantic feelings towards the female Inquisitor.
Thus, in 546.M32 the byzantine politics of the Imperium of Man took a calamitous turn when the High Lords of Terra were all assassinated. This lamentable event came to be known as "The Beheading." Vangorich then moved swiftly to install new High Lords, who acted as a puppet Senatorum Imperialis for the Lord Protector to do with as he willed.
Though he did not say as much, Vangorich believed that the acting Lord Commander of the Imperium had more or less given unspoken permission to the Lord Protector to remove the chafe from the wheat. Whilst out on Crusade, Thane received reports of what Vangorich had done, but did not move against him, as he decided to let the Lord Protector lead the newly-installed Senatorum Imperialis and rule the Imperium effectively for once.
Vangorich would go on to govern the Imperium for nearly a standard century. Though he eventually became a tyrant, he was an effective one, and oversaw the reconstruction and re-fortification of Terra and the Imperium, the rebuilding of Imperial military forces as well as the Fourth Founding of the Adeptus Astartes. However, after nearly eighty Terran years of rule, the Lord Protector had begun to display a variety of troubling behaviours and uncontrolled paranoia.
Seeing enemies everywhere, he called for unnecessary purges and massacres, which only seemed to grow in number on a daily basis. Receiving troubling reports from abroad about the Lord Protector's growing instability, Maximus Thane decided that the Imperial Fists would return to Terra to remove the troublesome Vangorich.
Fury of the Space Marines
When the Imperial Fists' Crusade fleet finally arrived back at Terra, four hundred Space Marines were despatched to the capital world's surface. A company each from two of the newly founded Chapters, the Halo Brethren and the Sable Swords, as well as two hundred Imperial Fists of the 1st, 4th and 5th Companies took part in the efforts to remove Vangorich from power. The commander of the strike force, Qublicus Amar, Chapter Master of the Sable Swords, was assassinated by a Vindicare Assassin's bullet soon after making planetfall.
Seeing no other choice, the remaining Space Marines commenced their attack nevertheless. Assailed by hundreds of Assassins, the Space Marines lost half their number securing the Imperial Palace from the agents of the Officio Assassinorum. Confronting the Inquisitorial Representative, the elderly Wienand, she convinced Thane of her loyalty, and gave the Chapter Master the necessary intelligence he needed in order to find and remove the mad tyrant Vangorich from power.
The Grand Master of Assassins had taken refuge in the Eversor Temple at Terra's north pole. Moving the retribution force there at once, the Astartes proceeded with their attack. Within the Eversor Temple, the Space Marines were assailed by a hundred Eversor Assassins. Only Maximus Thane managed to survive, to reach the Grand Master and finally deliver the Emperor's judgement personally with his Bolt Pistol. In the aftermath of Vangorich's fall, the Imperium would proceed to descend into a period of political anarchy for some time.
This sorry situation would last for almost two more standard decades, until Chapter Master Agnathio of the Ultramarines gathered the support of his fellow Chapter Masters from all over the galaxy, and a combined armada of over 50 Chapters of Space Marines descended upon Terra for the first time since the end of the Horus Heresy.
Agnathio provided the warring factions of the Throneworld with an ultimatum to either cease fighting and form a unified government immediately or be purged as Heretics. The factions, unable to withstand the military might of the equivalent of an entire Space Marine Legion of old, quickly fell into line, and a new council of High Lords of Terra were quickly elected and order restored at the heart of the Imperium.
- Warhammer 40,000 Rulebook (6th Edition), pg. 169
- Warhammer 40,000 Rulebook (5th Edition), pg. 124
- The Beheading (The Beast Arises Book 12) by Guy Haley, Chs. 1-5, 9-17