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Achren is a fictional character in the fantasy series The Chronicles of Prydain, written by Lloyd Alexander.

In the novels, Achren is the former Queen of Prydain, who ruled many years as a harsh and vengeful tyrant, both as ruler and then as the Death-lord Arawn's consort. She originally ruled from her citadel in Annuvin, near Mount Dragon, and later moved to the fastness of Spiral Castle after Arawn gained power. She secretly despises the Death-lord, who stole from her the crown of Annuvin. She is also taken with Gwydion; even though he does not reciprocate her feelings, she remains largely loyal to him after losing her powers.
Achren is a powerful enchantress, and was apparently the one who taught Arawn to use his own powers before he became more powerful than she. Although her powers had diminished, she still was capable of much sorcery when the companions encountered her on their travels.

In The Book of Three, Prince Gwydion and the Assistant Pig-Keeper Taran are captured by Achren and taken to her abode in Spiral Castle. The Princess Eilonwy had been living at Spiral Castle under the pretense that Achren was teaching her to be an enchantress; although she is no relation to the princess, Achren has raised Eilonwy to believe that she is the niece of the former queen, something that Eilonwy remarks to Taran she (correctly) suspects is a lie. She imprisons Taran in the dungeon and takes Gwydion to Oeth Anoeth, where she tries to coax him into joining her and helping her to regain her throne.
In The Castle of Llyr, Achren returns having lost most of her power (because her stronghold was destroyed when Taran and Elionwy, in The Book of Three, removed Dyrnwyn from the castle when they escaped, causing the castle to collapse) and seeking vengeance. With the help of the treacherous Magg, Achren kidnaps Eilonwy, in the hopes of using the princess' connection to the power of the House of Llyr to augment her old powers. Her plan is foiled by Gwydion and she later tries to kill herself twice but Gwydion stops her, urging her to seek better purpose in life.
In Taran Wanderer, Achren appears briefly at the beginning of the story: she has apparently turned away from evil and taken refuge in Caer Dallben at the invitation of the enchanter. By her own choice she keeps house and cooks for the others on the farm.
In The High King (winner of the Newbery Medal), the sword Dyrnwyn is stolen and the companions plan to regain the flaming sword. Achren sets out on her own to seek revenge on Arawn before the companions have a chance to restrain her. When she is attacked and left for dead by Magg (now working for Arawn) and later wounded by a number of Gwythaints, Taran's band finds her close to death. After healing her wounds, she leads them past the dangers of Mount Dragon into Annuvin.
She meets her demise when Arawn, disguised as a snake, comes close to biting Taran; Achren throws herself to the ground and wrests Arawn away from Taran, and the serpent bites her on the throat. She later dies from the injury, but with the destruction of Arawn she finally finds peace. The companions carried her body into Arawn's great hall in all honors in gratitude for her final sacrifice.