Arianrhod is the daughter of the Welsh Goddess Don and the sister of Gwydion.
Gwydion was counselor to King Math who could only remain alive if his feet lay in the lap of a virgin at all times except when he led his armies into battle.
During one such battle the virgin who had held King Math's feet was raped, and so there was need for a replacement.
Gwydion recommended his sister, Arianrhod. King Math put her virginity to the test by asking her to step over his magic wand.
As she stepped over the wand she gave birth to a boy child with yellow hair. The child cried loudly, and Arianrhod, humiliated, ran for the door, dropping yet another small object on the ground in the process.
Before anyone could catch a glance at the object, Gwydion wrapped it and hid it inside a chest.
King Math then performed rites for the yellow haired boy child, naming him Dylan. Dylan immediately ran for the sea and received the sea's nature and was never seen again.

A time later Gwydion presented Arianrhod with the object that he had hidden in the chest - a second boy child.
Arianrhod was outraged at the "evidence" of her humiliation at the hands of King Math and rejected the child.
She laid on him three curses:
He shall have no name except one she gives him.
He shall bear no arms except ones she gives him.
He shall have no wife of the race that is now on the earth.

Gwydion was outraged by these curses and worked to break them. He disguised himself and the boy child as shoemakers and traveled to Caer Arianrhod. When Arianrhod went to have shoes fitted, the boy child threw a stone at a bird and deftly hit it.
Arianrhod commented on the child's skillful hand. At that Gwydion revealed himself and the child and stated that she had just named him - Llew Llaw Gyffes, the Shining Skillful Hand.
This threw Arianrhod into a firey rage and she stormed back to Caer Arianrhod swearing that the boy would never bear arms or have a human wife.

Again Gwydion tricked Arianrhod into breaking her own curse. He disguised himself and Llew as travelers and sought refuge in Caer Arianrhod. While they were there Gwydion caused an illusion showing a powerful armada of ships advancing on Caer Arianrhod. Making ready for battle Arianrhod threw open her armory and armed her retainers. Gwydion suggested to Arianrhod that she give arms to him and Llew (still in disguise) and they would fight at the defense of the castle. She readily agreed and thereby, unwittingly, granted arms to her son, breaking the second curse.
Gwydion then revealed themselves to Arianrhod and told her that she may as well take the arms back from her son, as there really was no battle to be fought.
Enraged at being tricked a second time, Arianrhod took comfort in her third curse - that Llew would have no human wife.
Gwydion, upset at the cruelty Arianrhod was showing her son, vowed to break this curse also. Gwydion went to King Math and explained Llew's plight.
Combining their magic they created a woman made of flowers, Blodeuwedd, to be wife to Llew, and broke Arianrhod's third curse.

Humiliated by King Math, thwarted by her son, forsaken by her brother, Arianrhod retreated to her castle Caer Arianrhod. Here she later drowned when the sea reclaimed the land.