The Burning of Prospero
Before the outbreak of the Horus Heresy tension had run high during the Great Crusade about the use of psychic abilities by the Imperium. Kept ignorant about the Warp and Chaos, the Astartes legions did not understand the "sorcery" practiced by so-called "psykers," be they mortals or even their fellow brothers. In particular the psyker Primarchs Magnus the Red and his sorcerous legion, the Thousand Sons, were viewed with suspicion and distrust. Among their fiercest critics were the Space Wolves, whose culture of aggressive violence and savage tactics contrasted with the studious, devious nature of the Crimson King and his occult-obsessed disciples.
At the Council of Nikaea the Emperor, father of the Primarchs and sovereign of humanity, decried that psychic powers were proscribed by the Imperium. He kept hidden the truth about the Warp and the Dark Gods, as there was sufficient prejudice against psykers that his edict was readily accepted by most. The biggest losers from the decision were the Thousand Sons, who retreated to their homeworld of Prospero to continue their practices in secret.
Magnus, however, was keen to demonstrate the utility of his gift. He learned that Warmaster Horus Lupercal was fated to rise up in revolt, and so he mentally projected himself from Prospero to the Emperor on Terra. Unaware the Emperor was busy with his Great Work, attempting to marshal use of the Webway so humanity could forgo the need for Warp travel, Magnus caused a disaster through his manipulation of the Sea of Souls. Daemons poured into Terra itself, leading to a secret war within the Webway led by the Emperor against the forces of Chaos. Meanwhile, an order was sent to Leman Russ, Primarch of the Space Wolves, to arrest Magnus the Red.
At this point Horus Lupercal had indeed fallen to Chaos, but had not yet revealed his treachery. As Warmaster and thus commander of the Imperium's military, he contacted Leman Russ and convinced him to raze Prospero, kill the Thousand Sons, and put Magnus to death rather than merely place him under arrest. He even detached some of his own Sons of Horus legion to take part in the operation. In addition to these, Adeptus Custodes (the Emperor's personal force) and the Sisters of Silence (an order dedicated to hunting psykers) also accompanied the Space Wolves, who still made up the majority of the attack force.
Regretting what he had done, Magnus passively waited on Prospero for his punishment, and even blinded his legion to the coming of the Wolves so they would not resist. Thus the Wolves were able to attack with complete surprise some of the greatest seers in the galaxy. The spires and pyramids of Tizca, the capital city of Prospero, came under orbital bombardment, and then were subjected to drop pods filled with bloodthirsty sons of Russ. When they saw the changes the Warp had done to the flesh of the Thousand Sons, giving them all sorts of profane mutations, it confirmed their beliefs that Magnus and his sons had fallen into something truly infernal and evil.
Ultimately Magnus and Leman Russ dueled, with the Wolf King emerging victorious. Before he could make the killing blow, though, Magnus cast a spell that transported him and his surviving Sons, a little over a thousand Astartes, to the Planet of the Sorcerers, a sanctuary world within the Warp. Meanwhile, the Wolves left Prospero a barren ruin, mountain ranges crumbled, entire oceans boiled into vapor. It was only when Horus' treason became widely known after the Drop Site Massacre on Isstvan V not long after that Russ and the Wolves realized they had been manipulated, much as Magnus himself was despite his good intentions. The Space Wolves and the Thousand Sons would fight on opposite sides during the Heresy, but the hatred and rivalry between these former brothers stems more from the fate of Prospero than Horus' fall to Chaos.
