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.
The Book of Three
Princess Eilonwy and Taran found Dyrnwyn in an underground barrow of the dead, during their escape from Spiral Castle. The interred body of a king held the sheathed blade. The sword's removal caused the castle to collapse.
Dyrnwyn was sheathed in a black scabbard inscribed with ancient writing, which twined about the hilt and scabbard. Much of the writing was scarred and obscured. 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.
The High King
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.
The Foundling and Other Tales of Prydain
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 to come free of its scabbard, deeming Rhitta unworthy to wield it; when the heartless king forced the blade free, its power struck him dead. He was interred in the barrow in which Taran and Eilonwy, many years later, found him entombed.