Evnissyen felt slighted because he had not been consulted about the marriage. In spite, he mutilated and killed Matholwch's horses. Bran tried to smooth over the affair by replacing the animals with gold and silver.

Branwen sailed back to Ireland with Matholch. But the King's relatives demanded that he take revenge on Branwen for the incident with the horses. She was banished to the kitchens and the dirtiest of work. After a year she had a son Gwern.

A long time went by before Branwen managed to get a message to her brother by fastening a letter to a bird's leg. The Welsh invaded Ireland to avenge the insult, leaving Bran's son Caradawc (strong one) in charge while they were gone.

Bran, a giant, waded across the seas and forced the Irish to negotiate. The Irish agreed to turn the kingdom over to Branwen's son Gwern. At the crowning ceremonies, while the little boy was meeting Bran and his other relatives, Evnissyen grabbed him by the feet and threw him into a fire, killing him.

A great battle broke out. The Irish had an advantage, the cauldron of rebirth, a wedding gift from Bran to Matholwch. They lit a fire under it and threw in their dead warriors to revive them. Evissyen decided he had better redeem himself or Bran would kill him when the battle was over. Evnissyen, also a giant, hid among the bodies in the cauldren of rebirth. Stretching himself out completely caused the cauldren to burst. This act of atonement cost Evnissyen his life.

The welsh won the battle, but not without consequence. Only Branwen and a few men surrvived: Pryderi, Manayddan, Gluneu son of Taran, Talsesin the Bard, Ynawc, Grudyen son of Muryel, and Heilyn sonb og Gwynn the Ancient.

Bran was mortally wounded when a poisened dart punctured his foot. He had ordered that, when he died, his head be cut off and buried on the White Mount in London with his face towards France. Branwen subsequently died of a broken heart.