Baron Vardus Praal was the Imperial Governor of the Isstvan system, ruling from the Choral City on Isstvan III. His rebellion against the Imperium would precipitate the first major battle of what would become the Horus Heresy.
Praal had declared Isstvan's independence from the Imperium; the Emperor, seeking to prevent other star systems from following suit, dispatched Warmaster Horus to quell the rebellion, unaware that his favored son had been corrupted by Chaos.1 Under the Warmaster's command were his own Sons of Horus, the World Eaters, the Emperor's Children and the Death Guard. Horus, as part of his master plan for the coming war, sent select squads from all four Legions to the surface, under the pretext of crushing the rebellion at its source.
During the attack on the Choral City, Praal, surrounded by elite Warsinger bodyguards, met Emperor's Children Captain Lucius in combat. Despite his enhanced abilities (likely gifts from the Chaos Gods), Praal fell to Lucius' blade.2