In Welsh mythology, Dyrnwyn or White-Hilt was a powerful sword belonging to Rhydderch Hael, one of the Three Generous Men of Britain mentioned in the Welsh Triads. It was sometimes referred to as "Dyrnwyn, gleddyf Rhydderch Hael" or "Dyrnwyn, the sword of Rhydderch". When drawn, it blazed with fire; if drawn by a worthy man, the fire would help him in his cause, but its fire would burn the man who drew it for an unworthy purpose. Dyrnwyn was also one of the Thirteen Treasures of the Island of Britain. It is possible there is a connection between this sword and Owain Dangwyn, a ruler of Rhôs. His second name means "white tooth" which perhaps refers to this sword. Owain Dangwyn has been associated by some scholars with King Arthur due to several coincidences in his life with the legendary figure including; being murdered by his nephew (Maelgwn Gwynedd); having "ab Yrthr" as his patronymic (his father was Einion Yrth); living at around the right time and he may have held the title "Artorius" meaning "the bear". There is further evidence of him ruling, for a time at least, from Viroconium where archaeological evidence indicates building work at this time. So, it is possible that this sword was the basis for the Excalibur legends.

Fiction

Dyrnwyn appears in Lloyd Alexander's Chronicles of Prydain as the sword of Prince Gwydion. The sword was made by Govannion the Lame, by command of King Rhydderch Hael, and a spell was cast on it to protect it from misuse. A warning was engraved on the scabbard: "Draw Dyrnwyn, only thou of royal blood, to rule with justice, to strike down evil. Who wields it in good cause shall slay even the lord of death." It was the most powerful weapon in Prydain, but disappeared mysteriously with King Rhitta, the grandson of Rhydderch.

In the later books, it was revealed that the warning on Dyrnwyn's scabbard had been misread. The phrase "royal blood" should have been translated as "noble worth".

In The Book of Three, it is discovered by Taran and Eilonwy when they escape through the tunnels beneath Spiral Castle, and Eilonwy carries the sword until giving it to Prince Gwydion at the end of the book. It remains in his possession during the next three books of the Chronicles of Prydain, until it is stolen by a band of the Huntsmen of Annuvin at the beginning of The High King. Near the end of the book, Taran discovers it under a rock at the top of Mount Dragon, and uses it to kill the previously deathless Cauldron-Born. He then uses the sword to slay Arawn, and at that point the sword's magical powers vanish, its purpose fulfilled.

Dyrnwyn also appears in Karen Anne Webb's Adventurers of the Carotian Union series. It is introduced in The Chalice of Life, the first book in the series, as the only artifact powerful enough to free Eliander, the prince whose rescue is the object of the series' overall quest. In this series, the sword actually is the Dyrnwyn of Celtic legend, and its description derives directly from those given in Celtic sources of the blade that was part of the Hallows of Britain. Its fictional element references the "hiding" of the Thirteen Treasures of Ancient Britain. In the Carotian "universe", items from caches like the Thirteen Treasures were too powerful to remain resident in the material world for very long. They were assigned other-worldly guardians who moved them around in time and space and delivered them into the hands of trustworthy people for specific purposes (in this case, the release of Eliander from a plane of incarceration that resides outside normal space-time). Other Celtic or Arthurian influences appear in the series: a reference to the woodland god Herne the Hunter as the sword's guardian, for example, and Eliander as a figure similar to Arthur as the "king that was and will be."