Claudas was born into a broken inheritance. His family held lands around Bourges and Berry in central Gaul, but they were vassals of King Aramont of Brittany, a powerful overlord of the Old Frankish world. When Claudas broke faith with Aramont and transferred his allegiance to the King of Gaul and Rome, Aramont joined forces with Uther Pendragon, the High King of Britain and father of the future King Arthur. Together they invaded Claudas's territory and laid it waste so thoroughly that only the city of Bourges was spared. The devastated region became known as La Terre Deserte, the Land Laid Waste, and Claudas himself was driven into exile. He remained in hiding until the deaths of both Uther and Aramont, and then he returned to reclaim his ruined kingdom with a vengeance that would reshape the Arthurian world.

The defining conflict of Claudas's life began with his neighbours to the north-west, the brother kings Ban of Benoic and Bors of Gannes. Ban and Bors were among Arthur's earliest and most powerful allies, the Frankish kings who had crossed the sea to help the young Arthur defeat the eleven rebel British kings at the battle of Bedegraine. In return for their service, Arthur had promised to aid them against Claudas, who had begun making incursions into their borders. The three kings - Arthur, Ban, and Bors - defeated Claudas at the battle of Trebe, where Claudas had allied himself with Duke Frollo of Germany and the Roman Pontius Anthony. But the victory was temporary. When Claudas attacked again, Arthur was preoccupied with a fresh rebellion in Britain and could not send the reinforcements he had promised.

The fall of Benoic and Gannes was one of the great tragedies of the Arthurian Age. Claudas overran both kingdoms in a campaign of fire and sword. King Ban fled with his queen Elaine and their infant son, but as they escaped from the stronghold of Trebes, Ban looked back and saw his castle in flames. His heart broke, and he collapsed and died on the spot. Queen Elaine, rushing to her husband's side, left the infant unattended for a few moments, and in that gap the Lady of the Lake, the enchantress who lived beneath the waters of a hidden lake, swept the child away to raise him in her underwater palace. That child was Lancelot, who would grow up to become the greatest knight in the world. King Bors died fighting against Claudas's armies, and his sons Lionel and Bors the Younger were taken as prisoners to Claudas's court, where they were raised under the watch of Claudas's retainers. In a single campaign, Claudas had orphaned the three cousins who would one day destroy him.

Claudas's court became a cage for the sons of his enemies, but it was also the forge in which they were tempered. Lionel and Bors the Younger grew to manhood in Claudas's household, treated not as slaves but as hostages of rank, given training in arms and the manners of court. Claudas's own son Dorin grew up alongside them, but the three boys were never friends. Dorin became cruel and disliked, and when Lionel and Bors reached the age of fifteen, they rebelled against their captor and slew Dorin in a violent confrontation. Before Claudas could execute them in vengeance, a servant of the Lady of the Lake rescued the brothers and spirited them away to join their cousin Lancelot in the enchanted realm beneath the lake. All three grew up to be knights of surpassing skill, and all three would one day sit at the Round Table and turn their swords against the man who had orphaned them.

The final reckoning came years later, after Arthur had consolidated his kingdom and Lancelot had proven himself the finest knight in Christendom. The immediate cause was the imprisonment of Elyzabel, a cousin of Queen Guinevere, whom Claudas had seized when she travelled as a messenger to the Lady of the Lake. Arthur, Guinevere, Lancelot, Lionel, and Bors the Younger declared war on Claudas with the full might of the Round Table behind them. Claudas fought with every weapon at his disposal: he allied with Rome, he gave lavish gifts to win the loyalty of his barons, he allowed any who wished to leave the country to depart before the war began, and he placed his son Claudin and his commander Chanart in charge of his armies. In one battle near Gannes, Claudas wept rather than abandon his people to danger, showing the strange mixture of ruthlessness and tenderness that made him the most complex villain in the Vulgate Cycle. But he could not win. He refused to free Elyzabel even when Lionel, Dorin's killer, demanded her release, and that stubbornness sealed his fate. Defeated and stripped of all his conquests, Claudas slipped away to Rome, where he died in exile and disgrace. His former lands passed to Lancelot and his kin, and his son Claudin, who had fought honourably against the Round Table, was spared and later became one of the knights who achieved the Holy Grail.