Space Marines - Horus Heresy: At the very height of the Emperor's Great Crusade, the traitorous Warmaster Horus led his Traitor Legions of Chaos Space Marines against those who stood loyal at the Emperor's side.Hundreds of worlds burned in the name of the Dark Gods, and a terror unlike any seen before was unleashed across the galaxy during the seven dark standard years that the Heresy raged after the first battles in the Istvaan System.Much of the truth of these times has been lost, obscured by the mists of time or embellished to the point where giants bestrode worlds with thunderous steps and the planets themselves cracked and split at their tread.The Traitor forces of Horus drove all before them, until those Astartes Legions still loyal to the Master of Mankind stood at bay upon the fortified walls of the Imperial Palace during the climactic Siege of Terra.The forces of darkness pressed in around the guttering flame of Humanity, but desperate times called for desperate solutions. Sanguinius of the Blood Angels and Rogal Dorn of the Imperial Fists, together with their bravest warriors, decided to accompany the Emperor and take the fight to Horus upon his Battle Barge the Vengeful Spirit, a mighty Gloriana-class Battleship in orbit above Terra.The Emperor and His warriors teleported onto Horus' flagship but found themselves separated and scattered throughout the corrupt vessel by means of the Warmaster's dark sorcery.The Emperor fought His way to the Warmaster but was too late to save Sanguinius, who Horus slew when the angelic primarch refused to turn to Chaos. Yet, some maintain that Sanguinius inflicted a wound, however small, upon his erstwhile brother.Horus and the Emperor clashed in a battle of both flesh and spirit. Horus was filled with the power of the Ruinous Powers and dealt the Emperor a mortal blow, but in the end, the Emperor's will was the greater, and Horus was struck down with the last ounce of the Emperor's strength. The Archtraitor was destroyed utterly, in body and soul and, with his death, the power of the Traitor Legions was broken.When Dorn and his warriors finally fought their way into the rebel Warmaster's sanctum, they found the Emperor's broken and ruined body, and it is said that their cries of woe were heard far below on Terra itself.Rogal Dorn, most determined and unbending of the primarchs, bore his master's body back to Terra and, under the direction of the crippled Emperor, bound Him within the strange psychic augmentation device known as the Golden Throne to sustain His existence for all eternity with constant sacrifice and baroque machineries.The followers of the Ruinous Powers were defeated, but it was victory won at a terrible cost. The brotherhood of the primarchs was sundered, and the Emperor's vision for the Imperium and all of Mankind lay in ruins -- the last, best hope of a new golden age for Humanity lost forever.The galactic empire for Humanity the Emperor had forged was all but destroyed, and it was to take many more standard years of brutal warfare during the Great Scouring before all the Traitor forces were defeated and driven into the hellish chaos of the Eye of Terror.The death toll numbered in the billions, and uncounted worlds had been left as little more than corpse-haunted wastelands as the raging inferno of what Imperial savants later named the Horus Heresy was finally extinguished, though Mankind still teetered on the very brink of extinction.The Heresy had revealed weaknesses in the gene-seed of several of the early Space Marine Legions, which had been exacerbated by the need to keep the huge formations up to strength in the terrible wars being fought during the Great Crusade.The insidious powers of Chaos had been able to manipulate this corruption to turn Horus and many of the Space Marines under his command against the Emperor.Once Horus was defeated, it was decided by Roboute Guilliman, the primarch of the Ultramarines Legion who became the day-to-day ruler of the Imperium, that the forces of the Imperium would be reorganised so that a similar catastrophe could not be repeated.The Space Marine Legions were divided up to create one Chapter of the same name as the founding Legion and a number of new Chapters with new names. This critical event of the early 31st Millennium was called the Second Founding, and over two dozen further Foundings have occurred in the ten millennia since.It is not known exactly how many Chapters were created in the aftermath of the Horus Heresy, as many of the Imperium's records are undertsandably incomplete or lost entirely, and whole Chapters have been created and destroyed in the millennia that have followed.What is known is that there are just over a thousand Chapters scattered across the Imperium, each a brotherhood of the very finest warriors Humanity has ever called to its service.