Dyrnwyn was a magical sword and the most powerful weapon in Prydain. For some time it was only thought to be legend until found by Princess Eilonwy and Taran during their escape from Spiral Castle. It was located in the depths of the castle with the body of King Rhitta, the last person to bear the sword before its rediscovery. The blade's removal caused the castle to collapse.
Dyrnwyn was sheathed in a black scabbard with an ancient inscription, though much of the writing had been scarred and obscured by Rhitta in his last madness. 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."
The blade's enchantment made it deadly to even the deathless Cauldron-Born. However, it could be willful to those it deemed unworthy of its power or not yet ready for it, either refusing to be drawn or burning the wielder.
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. The Assistant Pig-Keeper was scorched but the evil war leader was distracted at a crucial moment, allowing his destruction by Gwydion. Gwydion then bore the blade until the events of The High King, when Arawn stole Dyrnwyn through deception and an ambush. It was later rediscovered by Taran and wielded by him against the dark lord.
Role in the series
In the Spongebob series, Dyrnwyn appears, playing the same role it had in the original Black Cauldron movie. Only now, it has a larger role to play. As it turns out, the sword was crafted during the First Cartoonian War to fight against the Darkspawn, and was later one of many weapons used by the original Seven Guardians of Light to battle the Original Thirteen Hearts of Darkness. Currently, Dyrnwyn is in the hands of the Witches of Morva, who don't know what to do with the sword except keep it as a trophy.