Dinabuc first appears in Geoffrey of Monmouth's Historia Regum Britanniae around 1136, though Geoffrey never gives him a name. In Geoffrey's account, the giant is simply "a giant of monstrous size" who had emerged from Spain, snatched Helena, the niece of Duke Hoel of Brittany, and fled with her to the top of Mont-Saint-Michel. Hoel was one of Arthur's most loyal allies, the king of Brittany who had aided Arthur in his wars against the Saxons, and his grief was beyond measure. The giant had killed or captured every knight who came to rescue Helena, eating some while they were still half-alive, and his cave on the mount had become a place of horror. When Arthur, already embarked upon his campaign against the Roman Emperor Lucius Hiberius, heard the news, he did not hesitate. He took only two companions, Sir Kay, his seneschal and foster-brother, and Sir Bedivere, his loyal marshal, and set off for the mountain.

The story was expanded and given emotional weight by Robert Wace, a Norman cleric writing his Roman de Brut around 1155. Wace gave the giant the name Dinabuc, though he otherwise calls him simply "gaianz," giant, or "le munstre," the monster. In Wace's version, Helena's nurse survives and tells the tale of horror to Bedivere when he discovers her weeping beside a newly made tomb. She explains that when the giant seized Helena, the maiden was so terrified that her spirit fled her body before the monster could defile her; she died of fear rather than submit. But the giant, inflamed with lust, turned upon the nurse herself and violated her by force. The nurse warns Bedivere to flee, for the giant is a devil without pity, but Bedivere returns to Arthur with the tale, and the king's resolve hardens into steel. Arthur dreams before the battle of a bear and a dragon fighting, with the dragon victorious, and his wise men interpret it as the struggle between Arthur and the giant, with the dragon representing the king's triumph.

Arthur and his companions approach the mount by night and see two fires burning, one to the east and one to the west. Arthur sends Bedivere to investigate the smaller fire, where he finds the nurse and the tomb, while Arthur himself, with Kay, goes toward the greater blaze. They find Dinabuc sitting before a roaring fire, roasting flesh upon a spit and cutting slices from the side that is most cooked. The giant is so enormous that his club is a great branch of oak, a burden for a mighty man, and his hide is tough as crocodile skin. Arthur, determined to fight honourably, does not strike the monster while he sleeps, though he could have ended the battle in a moment. Instead, he approaches openly, sword drawn, and calls the giant to account for the death of his kinswoman. Dinabuc leaps up, seizes his burning club from the fire, and attacks with the fury of a cornered beast.

The combat is brutal and prolonged. Arthur is nimble and skilled, but the giant's strength is overwhelming. Dinabuc strikes at Arthur with the flaming club, and the king leaps aside, dodging blows that would shatter a mortal man. Arthur strikes back with his sword, and in one telling blow he slices between the giant's eyebrows, blinding him with a torrent of blood that pours down his face. Dinabuc, now half-blind, thrashes about in fury, groping for Arthur with his massive hands. He catches the king by the arm and nearly crushes him, but Arthur wrenches free with great pain. The giant stumbles over his own club, falls, and rises again, groping until he sees the shadow of the king. He springs at Arthur, catches him by the flanks with both arms, and squeezes so hard that he nearly breaks the king's spine. But Arthur, in a desperate stroke, thrusts his sword upward beneath the giant's crocodile skin armour, piercing the hide that no blade should have penetrated, and drives the weapon deep into the monster's vitals. Dinabuc collapses, dead upon the ground, his reign of terror ended.

Sir Kay beheads the giant's corpse, and the head is carried back to Arthur's camp, where the knights crowd around to gaze upon it and praise the king who had freed the country from such a voracious monster. Hoel mourns his niece and orders a chapel built above her grave on the mountain peak. In Layamon's Brut, the Middle English version composed around 1200, the giant has no name at all, and Layamon never uses the word "giant," preferring Old English terms like "eotend" and "feond," fiend, to emphasise the creature's demonic nature. The Alliterative Morte Arthure of the 14th century retells the episode with different details, and the Parlement of the Thre Ages transforms the adversary into a dragon rather than a giant. But in every version, the core remains: Arthur, the king, faces a monster alone, risks his life for a kinswoman he cannot save, and sanctifies the place of battle with a church that endures to this day. The abbey of Mont-Saint-Michel, rising from the tidal flats of Normandy, stands upon the site where Arthur killed Dinabuc, a monument to the victory of the dragon over the bear, of civilisation over monstrosity, and of mercy over the devouring maw.