According to Culhwch and Olwen, the hero Culhwch is cursed by his stepmother to love only Olwen, the daughter of the giant Ysbaddaden Bencawr. The giant sets Culhwch a series of impossible tasks, known as the anoethau, which must be completed before he can marry Olwen. The most difficult of these tasks is obtaining the comb, scissors, and razor from between the ears of Twrch Trwyth (Tro the Boar) - implements needed to cut Ysbaddaden's hair and beard, as the wedding ceremony requires the giant to be properly groomed. Recognizing that the task is beyond any single hero, Culhwch enlists his cousin King Arthur and his entire host of warriors.

Twrch Trwyth and his seven offspring (also enchanted beings) dwell at Esgeir Oerfel in Ireland. Arthur's army sails to Ireland to confront them. The hunt requires extensive preparation: Arthur's dog Drudwyn (the only hound capable of hunting the boar), a special leash, collar, and chain, and the divine youth Mabon ap Modron (the only one who can handle the hound). Menw the shapeshifter is sent ahead in the form of a bird to scout and, if possible, snatch the treasures from between the boar's ears. Menw is wounded merely by the boar's venomous bristles, demonstrating the creature's deadly nature.

Arthur's forces fight Twrch Trwyth for nine days and nights in Ireland but fail to kill him or retrieve the treasures. The boar and his seven offspring then swim across the sea to Wales, landing at Porth Cleis in Dyfed. What follows is a devastating rampage across south Wales, during which Twrch Trwyth kills cattle, destroys villages, and slays many of Arthur's finest warriors. The hunt passes through Dyfed and Ceredigion, between the Tawe and Ewias rivers, and to the Severn Estuary. Among the fallen are Gwarthegyd son of Kaw, Tarawc of Allt Clwyd, and even Arthur's own son Gwydre.

At the Severn, Arthur and his champions corner Twrch Trwyth in the water. Mabon ap Modron spurs his horse into the river and seizes the razor. Cyledyr the Wild takes the shears. Before they can retrieve the comb, Twrch Trwyth finds the riverbed with his feet and bolts away. The retrieval of these two treasures comes at terrible cost: Cacamwri is dragged under by millstones, and Osla Big-knife loses his weapon and nearly drowns.

Twrch Trwyth's physical description in the tale emphasizes his monstrous nature. He is gargantuan in size, far larger than any natural boar, capable of ravaging "a third of Ireland" in his destructive rampages. His bristles are "like sharpened iron" and release venom when shaken, inflicting lasting wounds. His tusks "gleamed like burnished silver." He can outpace horses, dogs, and even magical pursuers, and he can swim across seas. His seven offspring - Grugyn Silver Bristle, Llwydawg the Killer, Twrch Llawin, Gwys, Banw, Benwig, and one unnamed - are each nearly as dangerous as their father.

The hunt continues to Cornwall, where Twrch Trwyth makes his last stand. Arthur himself engages the boar in direct combat. Eventually, the comb is wrested from him, and Twrch Trwyth - wounded, alone (his seven offspring all slain), and enraged - leaps into the sea at Land's End and disappears into the depths, pursued by Arthur's hounds Aned and Aethlem. He is never seen again. Unlike most monsters in heroic literature, Twrch Trwyth is not killed; he escapes into the ocean, symbolizing the indomitability of nature and wilderness.

The Irish cognate Torc Triath ("Boar King of the Otherworld") appears in the Lebor Gabála Érenn (Book of Invasions), suggesting a shared Insular Celtic mythological tradition of an enchanted boar of supernatural significance. Archaeological evidence of Celtic pig cults includes ritual deposits of pig bones in sanctuaries and graves from the Iron Age, indicating the deep cultural significance of the boar in Celtic religion.