Brisen was born into the world of Corbenic, the castle of the Fisher King, a realm that existed on the border between the mortal world and the otherworldly domain of the Holy Grail. She served King Pelles, the last in a long line of Grail keepers whose ancestor had been wounded by the Dolorous Stroke, a blow that had laid waste to his kingdom and left him unable to walk or ride, forced to fish from a boat as his lands withered around him. Pelles knew the prophecies: that only a son of Lancelot and his own daughter Elaine would be pure enough to achieve the Grail and heal the Wasteland. But Lancelot loved only Guinevere, the queen of Britain, and would never willingly betray her. It was Brisen who found the solution.

The defining act of Brisen's life was the deception that conceived Galahad. Elaine, called Amite in some versions of the Vulgate and Heliabel in others, was the most beautiful woman in her father's kingdom and the maiden who bore the Holy Grail at the castle's evening meals. She had fallen in love with Lancelot when he rescued her from a scalding bath where Morgan le Fay had imprisoned her, but the knight's heart belonged entirely to Guinevere. When Pelles determined that the prophecy must be fulfilled, he turned to Brisen. The enchantress prepared a potion of such power that it could cloud the mind of the greatest knight in the world. She poured it into Lancelot's wine, and when he drank it his senses grew dim and his judgment failed. Then she placed upon Elaine's finger a ring that belonged to Guinevere herself, and by the magic of the ring combined with the potion, Elaine appeared to Lancelot as the queen he loved. That night, in a chamber at Case Castle, Lancelot lay with Elaine believing her to be Guinevere, and from that union Galahad was conceived, the knight who would one day achieve what no other could.

The morning after brought fury and tragedy. When Lancelot woke and saw Elaine beside him instead of Guinevere, he drew his sword and would have killed her where she knelt naked on the floor. But Elaine begged for mercy, telling him that she carried in her womb the son who would be the noblest knight in the world. Lancelot stayed his hand and kissed her, but his rage turned upon Brisen. He swore that if he ever found the enchantress who had tricked him, he would strike her down. Brisen, however, remained safely in Corbenic, beyond the reach of the knight whose honour she had destroyed. The child was born and named Galahad, and he was raised in secret by his aunt in a convent until the day he would come to Arthur's court and begin the quest that his own conception had made possible.

Brisen's second deception was even more devastating than the first. Years later, after Galahad had been born and Elaine had come to Camelot to show Lancelot his son, the Grail Maiden found herself scorned by the knight who would not acknowledge her. She complained to Brisen, and the enchantress promised that she would make Lancelot lie with Elaine again. That night, Brisen went to Lancelot and told him that Guinevere herself had sent for him. Lancelot, eager as always for the queen's summons, followed Brisen to a chamber where Elaine waited. At the same moment, the real Guinevere had also sent for Lancelot, and when he did not come to her room she went looking for him. She found him in bed with Elaine, talking in his sleep, and her fury was absolute. She told him she never wanted to see him again, and Lancelot, maddened by grief, leapt from the window and ran into the wilderness, where he lived for two years as a naked madman, eating raw meat and sleeping in ditches, his mind shattered by the loss of the woman he loved. Brisen had not merely tricked a knight; she had unmade the greatest mind at the Round Table.

The end of Brisen's story is silence. Unlike Morgan le Fay, who continues to appear throughout the Arthurian tradition as a constant antagonist, or Nimue, who imprisons Merlin and then becomes a force for order, Brisen vanishes from the narrative after the second deception. She is not punished, not pursued, and not redeemed. She simply disappears, her work done, the prophecy fulfilled, the damage irreversible. In Thomas Malory's Le Morte d'Arthur, she is called Dame Brusen, and her role is identical: the potion, the ring, the deception, and the second trick at Camelot. In the Post-Vulgate Cycle, the details are much the same. The enchantress of Corbenic is not a villain in the traditional sense - she acts in service of a prophecy, at the command of her king, and for the sake of a holy quest - but her methods are those of the darkest magic, the theft of a man's will and the destruction of a woman's happiness. Whether she is a servant of God working through dubious means or a sorceress whose power serves only her master's ambition, the texts do not say. What they say is that without Brisen, there is no Galahad, and without Galahad, the Grail is never achieved.