On being told, he is furious that his half sister has been given in marriage without his consent, and flying into a rage he mutilates the horses belonging to the Irish.

[2] Branwen is treated cruelly by her husband Matholwch as punishment for Efnysien's mutilation of the horses, though not before she gives birth to an heir, Gwern.

She tames a starling and sends it across the Irish Sea with a message to her brother and Brân brings a force from Wales to Ireland to rescue her.

Some swineherds see the giant Brân wading the sea and report this to Matholwch, who retreats beyond a river and destroys the bridges.

The Irish lords do not like the idea, and many hide themselves in flour bags tied to the pillars of the huge, newly built house to attack the Welsh.

In the ensuing war, all the Irish are killed save for five pregnant women who lived in Wales who repopulate the island, while only seven of the Welsh survive to return home with Branwen, taking with them the severed head of Bendigeidfran.

On landing in Wales at Aber Alaw in Anglesey, Branwen dies of grief[2] that so much destruction had been caused on her account, crying, Oi, a fab Duw!

Brân had commanded his men to cut off his head and to "bear it even unto the White Mount, in London, and bury it there, with the face towards France."

At Llanddeusant, Anglesey on the banks of the Alaw can be found the cairn called Bedd Branwen, her supposed grave.