Blodeuwedd's story belongs to the Fourth Branch of the Mabinogion, the collection of medieval Welsh tales known as Math fab Mathonwy, the tale of Math son of Mathonwy. Math was the king of Gwynedd, a magician of immense power whose feet, when he was not at war, had to rest in the lap of a maiden, a condition that made him dependent upon the purity of his foot-holder. The tale begins with the treachery of Gwydion and his brother Gilfaethwy, who tricked Math into going to war so that Gilfaethwy could assault Math's foot-maiden, Goewin. When Math discovered the crime, he married Goewin himself and punished the brothers by transforming them into animals - first deer, then wild boar, then wolves - forcing them to bear offspring together in each form before restoring them to humanity. Then Math needed a new foot-maiden, and Gwydion suggested his own sister, Arianrhod.

Arianrhod was put to a test of virginity. She was required to step over Math's magic wand, but instead of proving her purity she gave birth to two sons: Dylan, who immediately leaped into the sea and became a creature of the waves, and a second child whom Gwydion snatched up and hid in a chest. This child grew up to be Lleu Llaw Gyffes, the Fair-Haired One of the Skillful Hand. But Arianrhod, shamed and furious, laid three destinies upon him: he would never have a name except one she gave him, he would never bear arms except those she gave him, and he would never have a wife from the race of women on Earth. Gwydion tricked Arianrhod into giving Lleu his name by disguising them both as shoemakers and provoking her to comment on his skillful hand. He tricked her into arming him by conjuring a phantom fleet that made her believe war had come, so that she armed him to defend her castle. But the third destiny could not be circumvented by trickery alone. No woman of Earth could be Lleu's wife.

So Gwydion went to his uncle Math, and together the two greatest magicians in Britain took the flowers of the oak, the flowers of the broom, and the flowers of the meadowsweet, and from those blossoms they conjured up the fairest and most beautiful maiden that anyone had ever seen. They baptised her in the way that was done at that time, and they named her Blodeuedd, which means simply "flowers." She was given to Lleu as his wife, and for a time they lived together in what seemed to be contentment. But Blodeuedd had been created, not born. She had no childhood, no mother, no history, no choice. She was a wife before she was a person, and when Lleu left her alone to visit his uncles, she saw Gronw Pebr, a warrior and hunter from Penllyn, riding through the woods in pursuit of game. She invited him to stay, and they fell in love with a passion that Blodeuedd had never felt for the husband who had been given to her like a gift.

The lovers conspired to kill Lleu, but killing him was no simple matter. Lleu was protected by magic, and he could only be slain under conditions so bizarre that they seemed impossible: he had to stand with one foot on the back of a bath and the other on the back of a goat, with the bath beside a river and a thatched roof over it but no walls, and he had to be struck by a spear that had been forged for a year during the hours of Mass. Blodeuedd, using all her charm and cunning, persuaded Lleu to demonstrate this position to her, claiming she was worried for his safety and wanted to understand how he could be harmed. Lleu, trusting his wife completely, stepped onto the bath and the goat beneath the thatched roof. Gronw emerged from hiding and cast the spear. It pierced Lleu in the side, and instead of dying he transformed into a grievously wounded eagle and flew screaming into the night. Blodeuedd and Gronw took possession of his lands and ruled together, believing their rival was gone forever.

But Lleu was not dead. Gwydion searched for him across the land and found him perched in an oak tree, barely alive, his flesh rotting and maggots falling from his wounds. Gwydion sang three poems to summon the eagle down, then struck him with his magic wand and restored him to human form. Lleu was nursed back to health, and when he was whole again he demanded justice. Gronw tried to make peace, offering land and gold and silver, but Lleu refused everything except the right to cast the same spear at Gronw that Gronw had cast at him. Gronw asked his war-band to take the blow in his stead, but they refused, and for this cowardice they were called one of the Three Disloyal Warbands of Britain. Gronw stood where Lleu had stood, on the bank of the River Cynfael, and tried to hide behind a great stone. Lleu cast the spear with such force that it pierced the stone and Gronw both, killing the hunter instantly. As for Blodeuedd, Gwydion overtook her as she fled through the forest. He did not kill her. He did something worse. He struck her with his wand and transformed her into a bird. "Because of the shame thou hast done unto Lleu Llaw Gyffes, thou shalt never show thy face in the light of day henceforth, and that through fear of all the other birds. For it shall be in their nature to harass thee and despise thee wherever they find thee. And thou shalt not lose thy name, but shalt be always called Blodeuwedd." In the Welsh of that time, Blodeuwedd meant "flower-face," and it was also the name of the owl. By changing her name from Blodeuedd to Blodeuwedd, Gwydion fixed her forever in owl form, the flower-woman become bird-woman, condemned to fly by night and be hated by all other birds. The Mabinogion tells us that from that day to this, the owl has been called Blodeuwedd, and that all birds despise her.