The Mabinogi (or Mabinogion) is a collection of Medieval Welsh tales, mostly written down in the 14th and 15th centuries. It features tales of heroic exploits and romances, and is one of the many sources for the Arthurian legends. The first four sections of the book are called Pedair Gainc y Mabinogi 'The Four Branches of the Mabinogi', the last of which contains the following story of brotherly incest.
Math ap Mathonwy was lord of Gwynedd, a fine lord and a formidable warrior, but subject to a taboo that he could not live in peacetime unless his feet rested in the lap of a virgin. So he sent his trusted nephews, Gwydion and Gilfaethwy, to carry out his lordly duties in the cantrefs of Gwynedd. Little did Math know that Gilfaethwy had fallen deeply in love with his foot-bearer, a beautiful maiden called Goewin, and that Gwydion was plotting to get Goewin for his brother.
Gwydion was a man of magic and cunning. He set about stirring up war between Math and his powerful neighbour Pryderi and as soon as his uncle left with his armies to fight, Gilfaethwy broke into Math’s chamber and took Goewin by force.
Pryderi’s army was routed and chased across the north with great loss of life until a truce was called and it was agreed that Gwydion, the cause of the war, should face Pryderi alone. This he did and, using his powerful magic, he overcame Pryderi and killed him, winning the war.
And so Gwydion and Gilfaethwy returned to their circuit of Gwynedd and Math returned home to his court. But when he came to lay his feet in Goewin’s lap, she told him the truth: that she was no longer a virgin and that Gilfaethwy had taken her against her will. Math was furious and sent out a ban against his nephews denying them hospitality anywhere, so they were eventually forced to return to face him.
The two brothers stood before Math in shame and offered to make recompense for their treachery and deceit. Math said they could not compensate for the loss of life and the shame caused by their actions, and that he would punish them. Quickly, drawing his large wand, he strikes Gilfaethwy and turns him into a lithe hind. Gwydion, anticipating his own fate, tries to escape but is caught and transformed into an impressive stag.
—Since you have born a plot against me together, said Math —You will live together like wild animals and mate together and have young together like other beasts. And a year from now you will return to my court.
So the brothers ran from the court into the woods and lived as Math had said: they mated in autumn and the following spring Gilfaethwy gave birth to a sturdy stag fawn. Then at the end of summer they returned to the court where Math met them outside the gates. Recognising them he raised his wand, but instead of returning the brothers to their human form he made Gilfaethwy into a stout wild boar and Gwydion into a wild sow. Then he struck the fawn, who transformed to a strong, handsome, brown-haired boy who Math took in his arms and named Hyddwn before sending the brothers back to the woods to live as wild swine.
And so, once again, they lived in the woods for a year and mated, and this time Gwydion gave birth to a young boar-pig before they returned to the court at the end of summer. And there Math met them and struck them each in turn and Gwydion the sow became a fierce wolf, and Gilfaethwy the boar became a sleek she-wolf, and the piglet became a large red-haired boy. Math took the boy and named him Hychddwn before sending the brothers back to the woods to live as wolves.
There, for the third time, the brothers lived together and mated and in the spring Gilfaethwy gave birth to a strong wolf cub. Then as autumn neared they returned to the court where Math met them. This time he struck the cub first, turning him into a solid dark-haired boy who Math named Bleiddwn. Then he struck the brothers who shed their fangs and fur and transformed back to their human form, standing naked before their uncle and their three own sons. Math forgave them and welcomed them back to court where they watched their three boys grow into three fine young men, who became known as:
The three sons of wicked Gilfaethwy
Three true champions
Bleiddwn, Hyddwn and Hychddwn Hir
Math ap Mathonwy was lord of Gwynedd, a fine lord and a formidable warrior, but subject to a taboo that he could not live in peacetime unless his feet rested in the lap of a virgin. So he sent his trusted nephews, Gwydion and Gilfaethwy, to carry out his lordly duties in the cantrefs of Gwynedd. Little did Math know that Gilfaethwy had fallen deeply in love with his foot-bearer, a beautiful maiden called Goewin, and that Gwydion was plotting to get Goewin for his brother.
Gwydion was a man of magic and cunning. He set about stirring up war between Math and his powerful neighbour Pryderi and as soon as his uncle left with his armies to fight, Gilfaethwy broke into Math’s chamber and took Goewin by force.
Pryderi’s army was routed and chased across the north with great loss of life until a truce was called and it was agreed that Gwydion, the cause of the war, should face Pryderi alone. This he did and, using his powerful magic, he overcame Pryderi and killed him, winning the war.
And so Gwydion and Gilfaethwy returned to their circuit of Gwynedd and Math returned home to his court. But when he came to lay his feet in Goewin’s lap, she told him the truth: that she was no longer a virgin and that Gilfaethwy had taken her against her will. Math was furious and sent out a ban against his nephews denying them hospitality anywhere, so they were eventually forced to return to face him.
The two brothers stood before Math in shame and offered to make recompense for their treachery and deceit. Math said they could not compensate for the loss of life and the shame caused by their actions, and that he would punish them. Quickly, drawing his large wand, he strikes Gilfaethwy and turns him into a lithe hind. Gwydion, anticipating his own fate, tries to escape but is caught and transformed into an impressive stag.
—Since you have born a plot against me together, said Math —You will live together like wild animals and mate together and have young together like other beasts. And a year from now you will return to my court.
So the brothers ran from the court into the woods and lived as Math had said: they mated in autumn and the following spring Gilfaethwy gave birth to a sturdy stag fawn. Then at the end of summer they returned to the court where Math met them outside the gates. Recognising them he raised his wand, but instead of returning the brothers to their human form he made Gilfaethwy into a stout wild boar and Gwydion into a wild sow. Then he struck the fawn, who transformed to a strong, handsome, brown-haired boy who Math took in his arms and named Hyddwn before sending the brothers back to the woods to live as wild swine.
And so, once again, they lived in the woods for a year and mated, and this time Gwydion gave birth to a young boar-pig before they returned to the court at the end of summer. And there Math met them and struck them each in turn and Gwydion the sow became a fierce wolf, and Gilfaethwy the boar became a sleek she-wolf, and the piglet became a large red-haired boy. Math took the boy and named him Hychddwn before sending the brothers back to the woods to live as wolves.
There, for the third time, the brothers lived together and mated and in the spring Gilfaethwy gave birth to a strong wolf cub. Then as autumn neared they returned to the court where Math met them. This time he struck the cub first, turning him into a solid dark-haired boy who Math named Bleiddwn. Then he struck the brothers who shed their fangs and fur and transformed back to their human form, standing naked before their uncle and their three own sons. Math forgave them and welcomed them back to court where they watched their three boys grow into three fine young men, who became known as:
The three sons of wicked Gilfaethwy
Three true champions
Bleiddwn, Hyddwn and Hychddwn Hir