Constantinus, also known as Constantinus the Liberator, was a former squad Sergeant of the Sons of Guilliman Space Marine Chapter's 4th Company who turned to Chaos. He led the world of Nova Terra in a rebellion and subsequent costly and drawn out civil war against the Imperium of Man, known as the Constantinus Iconoclasm.
Originally a Sergeant in the 4th Company of the Sons of Guilliman Chapter, Constantinus and his squad fought to free the population of the world of Nova Terra from Tyranid attack. After driving the aliens off the world, his unit was left behind to monitor for signs of Genestealer infestation. For three standard years Constantinus led his squad on endless patrols through the caves of Nova Terra, cleansing the planet of the remaining pockets of Tyranid infestation. When rumour arose of a Genestealer infestation among the city of Cirtus' ruling aristocracy, Constantinus brutally hunted down the infected nobles and their families.
However the people of Nova Terra were enraged by the summary execution of their leaders and rose up in revolt. Disgusted by this "ungrateful" behavior, Constantinus destroyed entire sectors of the city of Cirtus, turning on those in his own squad who tried to stop his massacre. He appeared before the crowd and announced himself the new ruler of Cirtus and declared the Emperor of Mankind and His minions had turned him into such a monster. Constantinus vowed to lead the people of Nova Terra to freedom from the Imperium. Eventually commanding an army 3 million strong, they overthrew the planet's Imperial Governor. The Renegade Sergeant renamed the world of Nova Terra Constantinium and under his rule bloodthirsty mobs were given free reign, anarchy prevailed in the streets, and any dissent was viciously silenced.
Now uninhibited, the crowds eventually turned to worshipping the Gods of Chaos, which attracted the attention of Chaos Space Marine warbands. Constantinus fought and defeated the best Champions of Chaos, claiming their warbands for himself. He then embarked on a rampage across the neighboring star systems, and within a solar decade his wars had engulfed the entire surrounding sector. Eventually Imperial forces, including the Iron Knights and two other Space Marine Chapters, twelve Astra Militarum foundings, an entire battlefleet and agents of both the Officio Assassinorum and the Ordo Hereticus, arrived in the region to deal with the growing threat. Following a thirteen-standard-year-long war against the Forces of Chaos, the Imperium reclaimed the world of Nova Terra for the Emperor. The war itself only ended with the death of Constantinus at the hands of the agents of the Officio Assassinorum.
- Codex: Chaos Space Marines (4th Edition), pg. 19
- Codex: Chaos Space Marines (6th Edition), pg. 24
- Liberator (Short Story) by Jonathan Green
- Imperial Armour Volume Thirteen - War Machines of the Lost and the Damned, pp. 25