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."