Hellawes first appears in the Prose Lancelot, the vast romance that forms part of the 13th-century Vulgate Cycle, where she is called Lady Helaes of the Perilous Forest, Helaes the Beautiful, a one-night lover of Sir Gawain in an earlier adventure. The name may echo Héloïse, the famous lover of the medieval philosopher Abelard, suggesting that the romance writers saw in her a figure of tragic and forbidden passion. But it was Sir Thomas Malory who gave her the name Hellawes and made her the central figure of one of the most chilling episodes in Le Morte d'Arthur. In Malory's version, composed in the 15th century, she is the widow of Sir Gylbert the Bastard, a knight who had been slain in fair combat by Sir Meliot de Logres, a fellow knight of the Round Table. Gylbert's death was not the end of the affair, for Hellawes was a sorceress of terrible power, and she wrought a charm upon Meliot so that his wounds would never heal, bleeding ceaselessly until some knight dared enter the Chapel Perilous and retrieve the very sword that had killed Gylbert and a piece of the bloody silk cloth that wrapped his corpse in the chapel. Only then could Meliot's wounds be staunched and his life saved.

Lancelot came upon the sickly Meliot and promised to help him. He rode into a deep forest, following a black brachet, a small hound, that was tracking a trail of blood. The hound led him over an old and feeble bridge into a manor house, where he found the body of Sir Gylbert the Bastard lying in state, with his wife weeping beside him. Lancelot saluted the grieving woman and rode on, and soon he met a damsel who knew him well. She told him that her brother, Sir Meliot de Logres, lay near by dying from wounds that would not heal, and that only the sword and cloth from the Chapel Perilous could save him. Lancelot, who feared no peril, rode straight to the chapel. He tied his horse to a little gate and entered the churchyard. On the front of the chapel he saw many fair and rich shields hanging upside-down, shields he recognised from knights he had seen before. Then he saw thirty great knights standing before him, each more than a yard taller than any man he had ever seen, all clad in black harness, grinning and gnashing their teeth with demon-like countenances. Lancelot put his shield before him and took his sword in hand, ready for battle. But when he advanced resolutely upon them, they scattered on every side like dead leaves before the wind, giving him passage. The illusion had been tested and found wanting.

Inside the chapel, Lancelot saw no light but a dim lamp burning. He was aware of a corpse covered with a cloth of silk, and he knew at once that it was Sir Gylbert. He stooped down and cut a piece away from the cloth, and as he did so the earth seemed to quake beneath him, a tremor of supernatural power that would have sent a lesser man fleeing. But Lancelot was the greatest knight in the world, and he feared no enchantment. He saw a fair sword lying by the dead knight, seized it, and hurried out of the chapel. In the chapel yard the thirty black knights spoke to him with grim voices, demanding that he lay down the sword or die. Lancelot replied that whether he lived or died, no great words would make him yield it, and they scattered again before his courage. Beyond the chapel yard, on the highway, a fair damsel met him. She warned him to leave the sword behind or he would die for it, but Lancelot refused. Then she tried a different temptation: she begged him to kiss her but once. Lancelot refused again, saying that God forbade it, and that he saved his kisses for his love alone.

Then the damsel revealed herself as Hellawes the sorceress, Lady of Castle Nygurmous, and the truth of the chapel was laid bare. She confessed that the entire Chapel Perilous was an elaborate charade created for his sake and for Gawain's. She had loved Lancelot for seven years, but she knew that no woman could ever have his love except Queen Guinevere. Despairing of possessing him alive, she had hoped to possess him dead. If he had yielded the sword or kissed her, his life would have ended, and she would have taken his body, embalmed it, preserved it, and kept it with her for the rest of her days, kissing him and clipping him daily in despite of Guinevere and every other woman living. Lancelot replied only that Jesus preserve him from her subtle crafts, mounted his horse, and departed. Hellawes was left alone on the road, her magic broken, her love unrequited, and her hope of possessing even a corpse denied. The chronicles say that she died within a fortnight, not from any spell or wound but from pure sorrow, a necromancer who could command the dead but could not survive the death of her own desire.

The Chapel Perilous itself was an older motif than Hellawes. It first appears in the early 13th-century romance Perlesvaus, where the knight Perceval had to brave the horrors of a haunted cemetery and chapel to retrieve the shroud that had covered Jesus Christ. In the Vulgate Queste del Saint Graal, Gawain and Hector de Maris encountered a perilous chapel during the Grail Quest, seeing visions that portended the completion of the quest by Galahad, Percival, and Bors, and signifying their own unworthiness. The motif of the perilous chapel or perilous cemetery appears in various continuations of Chrétien de Troyes's Perceval, in Wauchier's and Manessier's versions, and in the romance Le Chevalier à deux Espées. But Malory's Hellawes is unique. She is not merely a guardian of a sacred place or a tester of knightly virtue. She is a woman of obsessive and unrequited love, a necromancer whose desire has twisted into something darker than seduction: the desire to possess the beloved even in death. Her name, Hellawes, echoes with the sound of hell and hawes, and her castle, Nygurmous, is the castle of necromancy, the black art of communing with the dead. She stands as one of the most disturbing figures in Arthurian legend, a reminder that the greatest perils are not always monsters or dragons but the human heart turned to darkness by love that cannot be returned.