Gorlois was a Duke of Cornwall and a proven warrior who had served under Ambrosius Aurelianus, the High King of Britain, in the wars against the Saxon invaders. He fought at the battle of Conisbrough against Hengist and his Saxons, and at Mount Damen against Octa and Eosa, earning a reputation as one of the bravest commanders in the British army. When Ambrosius died and his brother Uther Pendragon succeeded to the throne, Gorlois became Uther's vassal, a relationship that should have bound them in loyalty but instead became the fuse that lit the greatest tragedy in British legend.

The disaster began at a feast. Uther had called his nobles to court to celebrate his victories and to consolidate his new reign. There, for the first time, he saw Igraine, the wife of Gorlois. Geoffrey of Monmouth, the Welsh cleric who wrote the earliest complete account of Arthur's life around 1136, described Igraine as a woman whose beauty surpassed that of all the women of Britain. Uther fell instantly in love, and his love was not the patient or honourable kind. He made his desire obvious to the entire court, pressing his attentions upon Igraine in a way that no married woman could accept without shame. Igraine, faithful to her husband, told Gorlois what had happened. Gorlois, enraged and fearing for his wife's safety, gathered his people and left the festivities without asking Uther's permission, an act that Uther interpreted as treason.

Uther declared war on the Duke of Cornwall. Gorlois prepared his two strongholds for siege: Tintagel, the impregnable fortress on the rocky coast where the sea guarded three sides and a narrow path of rock offered the only approach, and Dimilioc, also called Terrabil, a castle further inland where Gorlois planned to make his stand. He placed Igraine in Tintagel, believing that no army could take it, and rode to Dimilioc to face Uther's forces in the field. The strategy was sound. Tintagel was virtually unassailable, and Dimilioc could be defended by a determined garrison. But Gorlois had not accounted for Merlin.

Uther's adviser Ulfin warned him that Tintagel could not be taken by force. The only way to reach Igraine was through the narrow path, and a handful of armed men could hold it against a vast army. Uther, driven mad with desire, sought out Merlin, the enchanter whose powers were already legendary. Merlin proposed a deception that would bypass every wall and every sword. He would use potions and hidden arts to transform Uther into the exact likeness of Gorlois. He would also disguise Ulfin as Jordan of Tintagel, a friend of the duke, and himself as Britaelis, another companion. While Uther's army besieged Dimilioc and drew Gorlois into battle, the three impostors would ride to Tintagel in the evening twilight. The sentries, seeing who they believed to be their lord and his friends, would give them entry. Uther would gain admittance to Igraine, and she, seeing only her husband, would give herself to him without suspicion.

The plan succeeded perfectly. Uther entered Tintagel and spent the night with Igraine, conceiving Arthur in the very bed of the man whose face he wore. He told Igraine that he had allowed Dimilioc to be besieged to draw the enemy away from Tintagel and keep her safe, and she believed every word. But while Uther lay with Igraine in the castle by the sea, the real Gorlois was fighting for his life at Dimilioc. Uther's army, led by commanders who knew nothing of the deception, pressed the siege with full force. Gorlois sallied out, was overwhelmed, and was killed in the battle. The messengers who brought the news to Tintagel arrived too late. Igraine had already given herself to the false Gorlois, and the true one lay dead on the field. When Uther revealed himself and claimed Igraine as his queen, she had no choice but to accept. She was pregnant with Arthur, and the child who would become the greatest king in legend was the product of a night of treachery, magic, and murder.

The consequences of Gorlois's death rippled through every generation of Arthurian legend. His daughters, raised in the shadow of their father's murder, became the agents of the kingdom's destruction. Morgause married King Lot of Orkney and later committed incest with her half-brother Arthur, producing Mordred, the traitor who would kill Arthur at Camlann. Morgan le Fay, the most powerful sorceress in Britain, nursed a hatred of Arthur that drove her to scheme against the Round Table for decades. Elaine married King Nentres and lived in the shadow of her sisters' fame. In Thomas Hughes's 1587 play The Misfortunes of Arthur, the ghost of Gorlois rose on stage to condemn the son of Uther, declaring that the sin committed against him would be avenged through Mordred, the hammer of his hate. The play made explicit what the legends had always implied: that Arthur's kingdom was built on a crime, and that the crime would demand payment in blood.