Once upon a time, Math son of Mathonwy was king over Gwynedd and Pryderi son of Pwll was king over many cantrefs in the south: the seven of Dyfed, the seven of Morgannwg, the four of Ceredigiawn, and the three of Ystrad Tywi. Math could not live unless he rested his feet in the lap of a maiden, save for when a war required him to leave. At that time, the maiden who was with him was Goewin daughter of Pebin of Dôl Bebin in Arfon, and she was the fairest maiden known in those parts.
Math had two nephews on whom he relied: Gwydion and Gilfaethwy, sons of Dôn. It was they who did the circuit of the land in Math’s stead, and the war-band went with them. It happened that Gilfaethwy became enamored with Goewin and was grown sick with lust after her. His brother Gwydion perceived the ailment of his brother and asked after its cause.
“I can’t admit to any that which ails me,” said Gilfeathwy. “You know that whatever words are shared between men, no matter how lowly they may be whispered, will reach his ears as soon as the wind meets them.”
“I know your thought: you love Goewin.”
Gilfeathwy heaved a heavy sigh, the heaviest sigh in the world.
“Sighing won’t give you what you want, but I have a plan to bring it about. There must be war for Math to leave the company of Goewin, so I will foment a war between Gwynedd and Powys. While Math is gone, then you may bed Goewin the fair.”
Then Gwydion went before Math and told him of a fabulous creature that was new in the land: the pig. He spoke of the sweetness of pork and how it was far above that of oxen, until Math desired to have pigs within his own kingdom.
“To whom belong the pigs, and how may they be bought?” asked Math.
“They belong to Pryderi, son of Pwll. They come from Annwn, and were a gift to Pwll from its king, Arawn. I’ll go to Pryderi and ask for the swine.”
“He very well may refuse you, my nephew, but do as you see fit.”
Gwydion dressed as a bard and journeyed down to Pryderi’s court in the company of a dozen other bards. Pryderi welcomed them, and called for Gwydion to tell a story, for he was dressed as the chief of bards. At that time Gwydion was the best storyteller in the world, and he entertained the whole court for that night and won the goodwill of Pryderi.
“Lord, I have not come only to entertain you and your court, but to ask of you those swine that you received as a gift from Arawn,” Gwydion said.
“I would gladly give them to you, but for an oath that I have taken. I swore that I would not give them away or sell them until they had bred double their number in the land.”
“If you exchanged the pigs, then you would not have given or sold them away, and thus your oath may be kept. Give me the pigs now, and tomorrow I will bring you an exchange for them.”
Pryderi agreed to exchange the animals once he had seen that the exchange would be worthwhile, and so Gwydion left to work his arts. He conjured beautiful horses and dogs with his magic, and turned toadstools into fine shields. In the morning, Pryderi gladly exchanged the pigs for the horses, dogs, and shields, which appeared to be so fine. He parted with Gwydion in friendship, not yet knowing he had been swindled, but Gwydion warned his companions that the spell would only last a day. They thus sped away to the north, for they knew Pryderi with his armies would soon march north to take vengeance upon them.
When they returned to Math, they warned him of the approaching armies. The pigs they placed within a sty in the fortress while the men of Gwynedd went out to meet Pryderi and his army, and Math went with them. Gilfeathwy remained behind and forced himself upon Goewin in Math’s chamber, and upon his bed. Though she cried aloud there remained no one to save her from Gilfeathwy’s lust. Meanwhile, the armies of Math and Pryderi clashed fiercely and there was a great loss of life. Pryderi and the southerners were finally pushed back and a peace agreement was reached. Pryderi gave hostages to Math, and began to retreat, but the two armies continued to harry one another with arrows.
Finally Pryderi sent word to Math that he wished to call off the two hosts and fight solely with Gwydion son of Dôn who was responsible for all the bloodshed. Both Math and Gwydion readily agreed to the terms and the two warriors met on the field. The battle between them was fierce and hotly contested, until by ding of his magic and strength, Gwydion overcame and killed Pryderi. Pryderi, son of Pwll, King of Dyfed and many realms in the south, was buried at Main Tryiawg, above Y Felenrhyd.
Then Math had mercy and returned the hostages to the host of southerners who were returning home in sorrow. When they returned to court, Math sought Goewin, and she was distraught.
“Lord, you must find yourself another maiden, for I have been made a woman against my will. While you were gone, your nephew Gilfeathwy raped me in your own bed, and though I fought and screamed, there was no one who could come to my rescue.”
“I will not send you away, but will make you my wife and authority over all my realms I will put into your hands.”
Math perceived what his nephews had done and was filled with a terrible wrath. He summoned them and pronounced his judgement upon them.
“You have dishonored me, and for this alone you should not live. But you are also responsible for the loss of a great many fine warriors, the rape of Goewin, and the death of Pryderi. Like animals you have behaved, and so animals you will become.”
Then Math struck them with his wand so that one became a stag and the other a hind and he banished them for a year. When they returned at the close of a year, the brothers had a fawn between them. Math turned the fawn into a young lad, who he kept and named Hyddwn. Then he struck Gilfeathwy and Gwydion again, turning the one who had been a stag into a sow, and the one who had been a hind into a boar. They were banished for a year and again returned at the year’s end with a piglet between them. The piglet Math transformed into another boy, who he kept and named Hychdwn. Then he transformed his nephews once more, one into a male wolf and the other into a female. At the close of a year, the brothers returned with a wolf pup between them. Math turned the pup into a boy, who he kept and named Bleiddwn.
Math saw that the brothers had been punished severely, as befitted their crimes, and that they had been humiliated be bearing sons by one another. Then he ended their torment and transformed them back into their usual forms. Then, in his mercy, he ordered them bathed and clothed and he extended his friendship to them once more.