In Welsh mythology, Blodeuwedd is the later name of Blodeuedd, a woman made from flowers. Her story is part of the Fourth Branch of the Mabinogion, the tale of Math son of Mathonwy.

Lleu Llaw Gyffes has been placed under three geasa by his mother Arianrhod, one of which is that he cannot have a wife of human kind. To circumvent this, Math and Lleu's uncle Gwydion create Blodeuedd from flowers and she marries Lleu.

Blodeuedd has an affair with Goronwy and they plot to kill Lleu. Lleu can only be killed under certain conditions, and Blodeuedd tricks him into telling her what these conditions are. He can not be killed indoors or outdoors, on horseback or on foot; and can be killed only by a spear forged when people are attending mass. Consequently he can only be killed whilst he had one foot on a bathtub and one on a goat (the bathtub being placed on a river bank but under a roof) and by someone using a weapon created as specified.

Under pretence of "Lord, will you show me how these conditions might be fulfilled..?", Blodeuedd conveys him to precisely this situation, with Goronwy lying in wait with the weapon. Lleu is (apparently) killed and Goronwy and Blodeuedd assume power. On hearing of this, Gwydion sets out to find and cure Lleu, who is now in the form of an eagle. Gwydion restores Lleu, who kills Goronwy.

Gwydion curses Blodeuedd, turning her into an owl. "You are never to show your face to the light of day, rather you shall fear other birds; they will be hostile to you, and it will be their nature to maul and molest you wherever they find you. You will not lose your name but always be called Blodeuwedd."

(Quoted from the Penguin Classic translation by Gantz to modern English: ISBN 0-14-044322-3.) Note the change in spelling is deliberate.