Dyrnwyn the Black Sword was an enchanted sword and the most powerful weapon in Prydain. It was forged long ago by Govannion the Smith for King Rhydderch Hael, and wielded justly by his descendants until it was lost during the reign of King Rhitta. For some time Dyrnwyn was believed to be merely a legend, even by the very wise.
Princess Eilonwy and Taran found Dyrnwyn in a tomb of the dead, during their escape from Spiral Castle. The body of a long-dead king held the sheathed blade. The sword's removal caused the castle to collapse.
Dyrnwyn had jewels studding its hilt and pommel, and a battered scabbard nearly black with age. Eilonwy noted that there was a "symbol of power" on the scabbard, a mark more potent than any Achren could have made, indicating a forbidden object. Entwined about the hilt and scabbard was an inscription in the Old Writing, much of which had been scratched away. When made clear by magical means, such as Eilonwy's bauble, the inscription read:
"Draw Dyrnwyn, only those of noble worth [sometimes mistranslated royal blood], to rule with justice, to strike down evil. Who wields it in good cause shall slay even the lord of death."
Eilonwy bore the weapon, undrawn, from its finding until the confrontation with the Horned King at Caer Dathyl, at which point Taran attempted to draw the blade. However, according to its inscription the sword could be dangerous to those it deemed unworthy of its power or not yet ready for it, either refusing to be drawn or burning the wielder. Taran tried desperately to free the blade from its sheathe but was only partly successful and had his arm badly scorched in the bargain. Yet the evil war leader was distracted at a crucial moment, allowing his destruction by Gwydion.
Gwydion then bore Dyrnwyn until Arawn stole the weapon through deception and an ambush. It was later rediscovered by Taran at the top of Mount Dragon. The blade's enchantment made it deadly to even the deathless Cauldron-Born; with it later Taran even slew Arawn Death-Lord himself. After this the white flame on Dyrnwyn was quenched, and it became an ordinary sword.
In the story "The Sword", it is revealed that King Rhitta, who had once ruled Prydain from Spiral Castle, was the last to wield the enchanted weapon. His coldness and lack of empathy for the suffering of his subjects caused the scabbard (and presumably the blade within) to blacken and stain. Rhitta even stooped so low as to strike a defenseless man, the common farmer Amrys, dead with its edge. Eventually the sword refused the King's hand, deeming hism unworthy to draw it. The ghost of Amrys haunted Rhitta, and at last when the raging king forced the blade free, its power struck him and his guards dead. King Rhitta lay as he fell, for he had bored mazelinke tunnels below his castle to escape the ghost, and his servants never found him. (Many years later, Taran and Eilonwy found Rhitta and his guards so entombed.)
The Disney movie presents Dyrnwyn as "the Magic Sword". During Taran and Eilonwy's escape from the Horned King's dungeons, they stumble upon the tomb of "King Gwydion" (as revealed in production materials), whereupon Taran finds the sword, believing the entombed king to have been a great warrior. Taran first uses the sword against an attacking guard, whose axe the enchanted blade melts. Taran goes on to use the sword to disarm guards, flood a room with brandy, and sever the chain that holds the drawbridge shut.
Taran wears the sword in its scabbard during the meeting with the Fair Folk, and wields it upon meeting Orddu, Orwen, and Orgoch when the witches attempt to prevent him from acquiring the Black Cauldron. Orddu greatly desires the weapon, leading the companions to trade it for the cauldron. The sword returns one last time at the end of the film, when Orwen offers it up as a fair trade for the Cauldron, against Orddu's protests. Taran declines the offer, and trades the Cauldron in exchange for the restoration of Gurgi's life.
Dyrnwyn is listed as one of the Thirteen Treasures of the Island of Britain, a series of items belonging to Welsh tradition. Otherwise known as "White Hilt," Dyrnwyn is said to be a powerful sword belonging to Rhydderch Hael, one of the Three Generous Men of Britain mentioned in the Welsh Triads. When drawn by a worthy or well-born man, the entire blade would blaze with fire. Rhydderch was never reluctant to hand the weapon to anyone, hence his nickname Hael "the Generous", but the recipients, as soon as they had learned of its peculiar properties, always rejected the sword.