Arianrhod (Welsh pronunciation: [arˈjanr̥ɔd]) is a figure in Welsh mythology who plays her most important role in the Fourth Branch of the Mabinogi. She is the daughter of Dôn and the sister of Gwydion and Gilfaethwy; the Welsh Triads give her father as Beli Mawr. In the Mabinogi her uncle Math ap Mathonwy is the King of Gwynedd, and during the course of the story she gives birth to two sons, Dylan Ail Don and Lleu Llaw Gyffes, through magical means.
According to the Fourth Branch, Arianrhod's uncle Math fab Mathonwy would die if he did not keep his feet in the lap of a virgin when he was not at war. Gilfaethwy conceives a lust for Math's original footholder, Goewin, and he and his brother Gwydion engineer a war with King Pryderi of Dyfed, forcing Math to leave his court. In his absence Gilfaethwy rapes Goewin, but is punished when Math returns (Math turns him and Gwydion into a series of mated pairs of animals). Math marries Goewin to alleviate her shame, but must find a new virgin to hold his feet.
Gwydion suggests his sister, Arianrhod. To test her virginity, Math tells her to step over his magician's rod. On doing this, however, she immediately gives birth to a young boy, Dylan Ail Don, and another little scrap of a thing that is not coherently visible, although Gwydion puts the boy in a chest in his bedroom. Dylan becomes a sea god and is accidentally murdered by his uncle, the metalsmith Govannon, but his brother is later weaned by Uncle Gwydion here. However, Arianrhod is still angry about her humiliation at Math's court. She places a tynged (a geis or curse) on the boy that he will never have a name unless she gives it to him. Gwydion disguises the boy as a shoemaker and returns to Caer Arianrhod; while Arianrhod is being fitted, she sees the boy killing a wren with a single stone and remarks that the fair-haired one ("lleu") has a skillful hand ("llaw gyffes"). Gwydion reveals the disguise, and says she has just given her son a name – Lleu Llaw Gyffes. Arianrhod then places a second tynged on Lleu, that he would never take arms unless she armed him. A few years later Gwydion and Lleu return to Caer Arianrhod, this time disguised as bards. Gwydion is an accomplished storyteller and entertains her court. That night, while everyone sleeps, he conjures a fleet of warships. Arianrhod gives them weapons and armor to help her fight, thereby dispelling her second curse. When Gwydion reveals the trickery, Arianrhod places a final tynged on Lleu: he would never have a wife from any race that is on this earth now. Gwydion and Math eventually break this curse by creating a woman out of oak blossom, broom, and meadowsweet; she is named Blodeuwedd ("flower face").
With her curses, Arianrhod denied Lleu the three aspects of masculinity: a name, arms, and a wife.
In other sources 
One of the Welsh Triads, 35 by Rachel Bromwich's numbering, establishes a different family connection for Arianrhod. Her father is named as Beli Mawr, and her brother is Caswallawn (the historical Cassivellaunus). She has two sons by Lliaws son of Nwyfre. Her sons, Gwenwynwyn and Gwanar, who both accompany Caswallawn in his pursuit of Julius Caesar, who has been chased from Britain. This triad is the only source connecting Arianrhod to Beli Mawr and the Caswallawn saga, but it is not incompatible with the tradition recorded in the Mabinogion. The stories of Welsh mythology changed over time, and the Mabinogion does not contain the only version of them. Welsh scholar William John Gruffydd noted that 15th- and 16th-century poets apparently knew an alternate tradition in which Arianrhod actually became Math's footholder. Additionally, some scholars have suggested that in an earlier form of the Fourth Branch, Gwydion was the father of Arianrhod's sons.
Arianrhod's palace, Caer Arianrhod, is connected with a rock formation visible off the coast of northern Gwynedd at low tide. This formation is one of several landmarks that attest to the localization of the events in the Fourth Branch in this area. The name "Caer Arianrhod" is also used in Welsh for the constellation Corona Borealis.
The name "Arianrhod" (from the Welsh arian, "silver," and rhod, "wheel") may be cognate with Proto-Celtic *Arganto-rotā, meaning "silver wheel." Alternatively, the earliest form of the name may have been Aranrot, in which case the first part of the name would be related to "Aran."
- Triad 35. Bromwich, Trioedd Ynys Prydein, pp. 284–285.
- MacKillop, Oxford Dictionary of Celtic Mythology, p. 24.
- Proto-Celtic—English lexicon and English—Proto-Celtic lexicon. University of Wales Centre for Advanced Welsh and Celtic Studies. (See also this page for background and disclaimers.) Cf. also the University of Leiden database.
- Bromwich, Rachel (2006). Trioedd Ynys Prydein: The Triads of the Island of Britain. University Of Wales Press. ISBN 0-7083-1386-8.
- Ford, Patrick K. (1977). The Mabinogi and Other Medieval Welsh Tales. University of California Press. ISBN 0-520-03414-7.
- Gantz, Jeffrey (translator) (1987). The Mabinogion. New York: Penguin. ISBN 0-14-044322-3.
- Ellis, Peter Berresford (1994). Dictionary of Celtic Mythology. (Oxford Paperback Reference) Oxford University Press. ISBN 0-19-508961-8
- MacKillop, James (1998). Dictionary of Celtic Mythology. Oxford University Press. ISBN 0-19-280120-1.
- Wood, Juliette (2002). The Celts: Life, Myth, and Art. Thorsons Publishers. ISBN 0-00-764059-5.