He took a band of men, including his brother, to Ceredigion, where they disguised themselves as bards to gain audience with King Pryderi.
Upon his return to his castle, King Math went to rest his feet in Goewin's lap, but could not, as she was no longer a virgin.
He took her as his wife to save her honour, and then as punishment, banished his nephews, turning them into a breeding pair of deer for a year.
Abandoning the child she fled in shame, letting drop a "lump of flesh" which was borne away by Gwydion and concealed in a chest.
Again, Gwydion disguised his nephew and himself, this time as bards, and like he did to King Pryderi, entertained Arianrhod's court with stories.
Together, they created a woman from the flowers of the oak, broom and meadowsweet and brought her to life, giving her the name Blodeuwedd "Flower-Faced".
One day, when Lleu was away from home visiting Math, Blodeuwedd saw a nobleman, named Gronw Pebr, passing and invited him to stay (for it would be rude not do so).
Blodeuwedd elicited from Lleu how he could be killed — with one foot on the back of a goat and the other on the rim of a bath of water, underneath a canopy, using a spear that had been made over the course of a year only on Sundays whilst the people were at Mass.
Gwydion went searching for his nephew and, by following a pig, found him (still an eagle) in a very bad state at the top of an oak tree by a lake.
Blodeuedd heard of this and fled, taking her maidens with her, but they were so frightened, that they ended up walking backwards to make sure nobody attacked them from behind.
The name Math is thought to derive from the Proto-Celtic *matu- meaning "good" or "fortunate" but also being a euphemism for a bear.