One of the sons of the goddess Don is Gwydion, "master of phantasy and illusion", the chief druid and magician of the Welsh pantheon. The people of Cymru, acknowledging his kinship with air and sky, called the Milky Way "the Castle of Gwydion".

In his youth, Gwydion resides with and serves as right-hand man to his uncle, Math ap Mathonwy, king of Gwynedd. He is also apprenticed to Math to learn magic. Gwydion's brother, Gilfaethwy, is also living at Math's court at Caer Dathyl. The two brothers assist Math by patrolling the grounds of Caer Dathyl with him, and by giving advice when he asks.

Math's special custom as king, at any time when he is not specifically at war, is to ever sit with his feet in the lap of a maiden foot-holder. During the period in which the story takes place, Math's foot-holder is a beautiful and high-born maiden named Goewin, daughter of Pebin of Dol Pebin in Arfon. Gwydion's brother, Gilfaethwy, develops a severe lust for her. Gwydion volunteers to help his brother satisfy it.

Conspiring together, the brothers decide to create a war so that Math will be lured away from Caer Dathyl. Gwydion tells his uncle Math that he has heard of some strange and miraculous creatures owned by king Pryderi of Dyfed -- small animals with flesh better than that of oxen. His curiosity aroused, Math allows Gwydion to go on a mission to secure the animals (swine; according to the myth, never before seen in Britain).

Gwydion disguises himself and Gilfaethwy as bards, and creates the appearance of ten more bards travelling with them, and under this pretense travel to Dyfed to speak with Pryderi. Taking the role of chief bard of their group, Gwydion entertains Pryderi's court with pleasant tales until he has the favor of all at court, including Pryderi.

Having thus secured the ear of the king, Gwydion broaches the subject of the herd of swine. Pryderi (of whom we have learned in past tales is a rash giver of gifts) hesitates at first, but later agrees to lend them in exchange for twelve black stallions, arrayed with the finest tack; twelve black greyhounds, and twelve golden shields.

During the night Gwydion conjures these things out of a toadstool. Purposefully, he designs the illusion so that it will dissipate after one day. Thus he plans to bring Pryderi into a war with Math.

Gwydion and Gilfaethwy drive the swine home to Gwynedd in haste, since they can hear the hosts of Dyfed only a day's march behind them. They inform Math that Pryderi is on his way to attack Gwynedd, and Math duly hastens out to assemble an army to defend the land, leaving Goewin unprotected. The two brothers force Goewin's serving-maidens away from her and Gilfaethwy makes his assault upon her in Math's own bed.

In the meantime the war with Dyfed has raged openly with severe casualties on both sides. Pryderi sends messengers to Math, informing him that his quarrel is chiefly with Gwydion, son of Don, whose magical trickery touched off the entire war. Gwydion agrees to fight Pryderi in single combat. Armed with magic and illusion as well as proficiency in war-craft, Gwydion slays Pryderi.

The war having ended, Math returns to Caer Dathyl, while Gwydion and Gilfaethwy remain away from court travelling about Gwynedd. Math soon learns of the injury done to Goewin in his absence, and issues a decree that no one in Gwynedd shall give Gwydion and Gilfaethwy meat or drink. Thus driven by necessity back to court, and tell Math, "Lord, we are at thy will."

"Had you been at my will, I should not have lost what I have of men and arms," Math replies sternly. "My dishonour you cannot make good to me, let alone the death of Pryderi. But since you are come unto my will, I will begin punishment upon you."

With a druidical wand, he strikes both Gilfaethwy and Gwydion, transforming them into a hind and a stag. "Since you are allied together, you shall live together and be coupled," he announces. "You shall also have offspring together. A year from today, you shall return to me."

Likewise after a year, Math transforms Gwydion into a sow and Gilfaethwy into a boar; and after another year, Gilfaethwy becomes a she-wolf, and Gwydion a he-wolf. The offspring -- fawn, piglet, and wolf-cub -- born to the two brothers are themselves transformed into human boys, given names, and fostered out.

After enduring these three transformations, Gwydion and Gilfaethwy are restored to their human shapes. "Men," Math declares to them, "if you did me wrong, long enough has been your punishment. Great shame have you had, that each one of you has borne young by the other." He orders baths made ready for his nephews, and orders them dressed and made ready for court.

Gwydion's next move is to introduce Math to his sister, Arianrhod daughter of Don, as a possible successor foot-holder to Goewin. In a tale full of unanswered questions and mystery, Gwydion brings about Arianrhod's very public embarrassment, and undertakes to raise her baby son. It is obliquely hinted at that Gwydion might be the father of her son. In any case Gwydion makes full use of his considerable magical powers on behalf of the child, and against Arianrhod, securing for him a name, arms, and eventually a wife, Blodeuedd -- all three of which are rights claimed by Arianrhod, and which she declines to give.

Gwydion remains close to Lleu Llaw Gyffes, for indeed this is the name he extracts from Arianrhod for her son. A trusting young man, Lleu is magically wounded by his wife's lover, Gronw Pebyr, and assumes the shape of an eagle whose flesh is rotting away. Gwydion, upon finding him sitting in a treetop in Gwynedd, dropping rotting flesh and maggots, sings him down from his tree by means of three englyns:

Grows an oak between two lakes,
Darkly shadowed by sky and glen.
If I speak not falsely,
From Lleu's flowers doth this come.

Grows an oak on upland plain,
Nor rain wets it, nor heat melts;
Nine score hardships hath he suffered
In its top, Lleu Llaw Gyffes.

Grows an oak upon a steep,
The sanctuary of a fair lord;
If I speak not falsely,
Lleu will come into my lap.

After seeing to Lleu's healing by the best physicians available, Gwydion sets about helping his nephew regain his lands. Pursuing Blodeuedd, he backs her up against a lake in which all her maidens drown. "I shall do worse than slay thee," he announces. "I shall let thee go in the form of a bird." The bird-shape he chose was that of the owl, and owls are called Blodeuedd to this day.

Source ~~ Argante's Guide to Celtic Deities