"Look upon my armor well, lap dog of the Corpse-Emperor, and tremble. For you knows these colors well, don't you? They were the colors that saw the Galaxy conquered, the colors that heralded death to three Legions on Istavaan, the colors that tore down the walls of old Terra. I am no coward, garbed in black. I am a Son of Lupercal, child of the Warmaster. And you, Loyalist... You are already dead."
―Vandahal the Sunderer, Legion Lord of the Sons of Lupercal
Hellish warriors, borne of a great schism that saw the Sons of Horus, now the Black Legion, split into countless fractious warbands claiming loyalty to one of two great Chaos Lords, the Sons of Lupercal are to many the old Luna Wolves reborn in the image of the Dark Gods. To others however, they are usurpers to Abbadon the Despoiler's rightful throne as heir to the XVIth Legion. The Sons of Lupercal and their client Warbands maraud across the Galaxy, forever prosecuting what they call the Long War against the Imperium of Man, continuing in earnest the Heresy their Gene-Father began so many eons ago. In this they have been amongst the most successful, achieving great gains while their erstwhile brethren skulk in the shadows, the Black Legion seemingly content to build up their forces within the Eye of Terror and mourn the loss of their Warmaster.
It was such sullen, impotent rage that saw Vandahal Goth, little more than a Sergeant within the elite Catulan Reaver squads suddenly rise to prominence and steal away with him over half of the remaining Sons of Horus in a bloody exodus. Many within the Legion despised Abaddon, the First Captain, right hand of Horus. They saw weakness in him ever since the rout from Terra, where he squandered their Primarchs sacrifice. Vandahal rose up, and in a hasty coup, attempted to assassinate the First Captain as he mourned at the shrine in which the Black Legion entombed their Primarch. Confronting Abaddon, decrying him openly in front of the whole gathered Legion as a fraud and coward, Vandahal and his closest followers attacked Abaddon and his Justaerin bodyguard. At first the Legionaries present were utterly shocked, but soon a brawl overtook those gathered, brother killing brother in the great shrine, supporters of both would-be inheritors to the Legion falling upon one another in an orgy of fratricide. In the end, Vandahal would take up the weapon of the Warmaster, Worldbreaker, and struck Abaddon down. Grievously wounded, the First Captain was dragged away from the field of battle, roaring promises of vengeance as the treasonous Sergeant fled with the Primarch's body in tow.
For several months the Black Legion tore itself apart in bloody civil war, fracturing into countless small Warbands, most pledging themselves to either Abaddon or Vandahal, though some would instead attempt to carve out their own piece of the Legion to lord over like petty kings. Eventually, the followers of Vandahal would come to call themselves the Sons of Lupercal, taking up the vestments of the Sons of Horus, and eventually fighting their way out of the Eye of Terror into the Galaxy with their supplicant Warbands. Now, a roving mass of hate and corruption, the Inheritors of the Warmaster wash across the stars, their Long War never ending, their thirst for vengeance unquenchable.
In keeping with the fractious and ad hoc command structures of the old Luna Wolves, the Legion itself makes up only roughly half of the overall strength of the Legion proper. Divided into various Warhosts, each the rough size of a traditional Great Company, the Sons of Lupercal number roughly half the strength of most of the remaining Loyalist Legions. However, the Sons of Lupercal are followed into battle by several large Warbands of Chaos Space Marines whose Chaos Lords have thrown in their lot with the Unmaker, either out of loyalty to his ideals or for a chance at greater power and glory alongside the mighty Inheritors of the Warmaster.