Brandin of the Isles first appears in the Vulgate Lancelot, the vast prose romance composed in the early 13th century as part of the Lancelot-Grail Cycle. He is introduced as the lord of Dolorous Guard, a castle on the River Humber that had once been called Joyous Garde before his enchantments made it a place of dread. The castle was protected by a terrible custom: any knight who wished to enter had to fight ten knights at the first gate and another ten at the second, one by one, until he had defeated them all. Only then could he face Brandin, the Copper Knight, in single combat. If the challenger failed, his name was carved on a gravestone within the castle walls, and a sculpted head was placed beside it, giving the impression that the knight was dead. In truth, the defeated were not killed but imprisoned in the Dolorous Prison, a dungeon in another of Brandin's castles, where they wasted away in darkness while their false graves deceived the world.

Brandin's power was not merely martial. He had been the paramour of Niniane, one of the Ladies of the Lake, the enchantress who raised Lancelot from infancy and who gave Arthur the sword Excalibur. Niniane had taught Brandin some of her magic, and he had applied these enchantments to his castle, making it a labyrinth of sorcery as well as steel. The Saxons had invaded Britain at the beginning of Arthur's reign, and Brandin had joined them against the young king, fighting on the side of the invaders who sought to tear the island apart. His allegiance to the Saxons made him an enemy of the Round Table from the start, and his castle became a symbol of everything Arthur's knights were sworn to destroy: tyranny, deception, and the misuse of magic for personal gain.

The crisis came when Lancelot, still a young knight who had only recently learned his true identity as the son of King Ban of Benoic, arrived at Dolorous Guard. He came seeking adventure and proof of his knighthood, and he found a castle that had defeated the bravest warriors in Christendom. The Lady of the Lake, his foster-mother, sent one of her damsels to aid him with three magical shields: one with a single red band that doubled his strength, one with two bands that tripled it, and one with three bands that gave him the strength of four men. Lancelot fought through the first gate, defeating six knights before night fell. The next morning, armed with the shields, he shattered the defences of the second gate. Brandin watched from the walls in his copper armour, leaning too far over the parapet in his terror. He tumbled from the wall and landed on one of his own knights below, killing the man instantly. The remaining defenders fled, and Lancelot entered the castle, though Brandin himself had escaped.

But Brandin was not finished. He gathered a hundred and fifty men and prepared a night raid against King Arthur's camp, intending to strike while the king's knights slept. Lancelot, warned by a hermit who knew of Brandin's plan, attacked the raiders single-handedly. Brandin's men panicked, believing that Arthur himself had caught them, and fled into the darkness. Lancelot captured Brandin and forced him to release all the prisoners from the Dolorous Prison, including Sir Gawain and his companions, whose names had been carved on the false gravestones. Brandin agreed, and the prisoners were freed. Yet even this humiliation did not break him. Disguised as a vavasour, a landless knight of low degree, Brandin invited Gawain, Yvain, and their companions to La Doloreuse Chartre, a second castle, where he intended to imprison them just as he had imprisoned the others. Lancelot saw through the deception, rescued his fellow knights, and forced Brandin to yield once more.

With Brandin defeated and his enchantments broken, Lancelot claimed Dolorous Guard as his own. He ventured into the depths of the castle, braved a final adventure in a cave beneath the chapel, and returned with the key that lifted the enchantments forever. The false gravestones vanished, the prisoners were freed, and the castle was restored to its original name: Joyous Guard. Lancelot made it his home, and in time it would become the refuge where he sheltered Queen Guinevere when Arthur condemned her to burn, the fortress where Arthur and Gawain laid siege and failed, and the place where Lancelot himself was buried beside his friend Galehaut. Brandin of the Isles vanished from the story after his defeat, his name remembered only as the tyrant whose fall made Lancelot's rise possible. In Thomas Malory's Le Morte d'Arthur, he appears as Sir Brian of the Isles, Nimue's evil cousin rather than her paramour, but the essence of the character remains: a knight of the bridge who guarded a crossing, attacked Queen's Knights out of hatred for Guinevere, and was defeated by the greatest knight who ever lived.