In Welsh folklore, the Cŵn Annwn or "Hounds of Annwn" ride through the skies in autumn, winter, and early spring. The baying of the hounds was identified with the crying of wild geese as they migrate and the quarry of the hounds as wandering spirits, being chased to Annwn. However Arawn himself is not referred to in these traditions. Later the myth was Christianised to describe the "capturing of human souls and the chasing of damned souls to Annwn", and Annwn was equated with the "Hell" of Christian tradition.
Some writers, notably Robert Graves, have written of an incident in which Amaethon steals a dog, lapwing and a white roebuck from Arawn, leading to the Cad Goddeu (Battle of the Trees), which Arawn lost to Amaethon and his brother, Gwydion. However the standard text of 'Cad Goddeu' in the Book of Taliesin makes no mention of Arawn and Graves based his interpretations on inaccurate early translations.
Arawn was the cause of much suffering throughout Prydain. He traveled throughout the land, always in disguise, attempting to steal the enchanted tools and recorded knowledge of Prydain's artisans and skilled laborers. The only man able to thwart him in this effort was Menwy the bard, the only mortal on record in Alexander's books to have seen Arawn's true face. Instead of using the stolen tools and knowledge for himself, however, Arawn kept them locked in his treasurehouse in Annuvin, where they would benefit no one.
His malignant influence had more direct effects as well. He captured the gwythaints and enslaved them with his power, changing them from relatively gentle birds into terrifying spies, the "Eyes of Annuvin". He also gained the Black Cauldron from Orddu, Orwen and Orgoch, corrupting it into the instrument that would create his Cauldron-Born warriors. Mortal men who joined with him became the Huntsmen of Annuvin, as fearsome as the Cauldron-Born in their own right. His powers, and the ramifications of them, proved seductive to many prominent leaders, including his champion the Horned King, King Morgant, Magg, and King Pryderi.
After his failure to steal the knowledge of the bards, Arawn rarely ventured beyond the borders of Annuvin, and then only in disguise. One of his greatest powers was the ability to transform into any shape, but this was also his greatest weakness, for he would then become as vulnerable as the creature whose likeness he assumed. Ordinary people were easily fooled by his disguises, but his former mistress Achren, who had taught him of all his powers, had the ability to see through his disguises.
Many of the events of The Book of Three and The Black Cauldron were triggered by Arawn's actions, but each time his efforts were thwarted. He struck again in The High King, personally showing himself to Prince Gwydion in the form of Taran, pursued by his own Huntsmen. Gwydion fell for the ruse, losing the sword Drynwyn and almost his own life as a result. With Dynrwyn in his possession, Arawn's allies rallied to his call, engulfing Prydain in an all-out war.
His own end came after sending all his Cauldron-Born to destroy Caer Dathyl, leaving Annuvin vulnerable. Following a strategy laid out by Gwydion, Taran and his companions approached Annuvin by land, fighting a delaying action against the returning Cauldron-Born while Gwydion led a larger force in a strike against Annuvin itself. With the help of a gwythaint Taran had befriended as a fledgling, Taran found the sword Dyrnwyn and used it to destroy the Cauldron-Born. He then entered Annuvin with the sword and quickly found Arawn alone, disguised as Gwydion. Arawn told him to give him the sword, which Taran almost did. But Arawn's mannerisms gave him pause, and after discerning the Death-Lord's true identity he tried striking him down. Gwydion's form blurred into a shadow and made a hasty retreat.
Minutes later Arawn returned to finish Taran off, approaching him and the other companions in the form of an adder. Achren saw him immediately and tried tearing him apart, but he struck and gave her a fatal bite on the throat. He then turned on Taran, who beheaded him with Dyrnwyn. After the snake expired it transformed into Arawn's human form, clothed in a dark robe with the head face-down. Just as quickly, Arawn's form faded away, leaving a patch of fallow land where it had been.
Arawn's death broke the powers of Annuvin. The Sons of Don and others with magical powers then departed Prydain, leaving the land in peace, as well as free of enchantment.