A King of Ireland to whom Bendigeid Vran, in an attempt to cement good relations between the two countries, married his sister Branwen. At the wedding feast, however, Matholwch was so insulted by Efnisien, half-brother to Bendigeid Vran, that he returned to Ireland, along with the magic cauldron given to him as a wedding present by Bendigeid Vran, and took his revenge on Branwen by treating her cruelly.
Bendigeid Vran learned of the mistreatment of his sister and mounted an expidition to Ireland to save her. At first Matholwch's forces had the upper hand, thanks to the cauldron Bendigeid Vran had given him, for the dead and wounded of the Irish forces could be restored to full vitality simply by dipping them in it. However, the tide turned when Efnisien managed to destory the cauldron. So complete was the destruction that Matholwch and his entire race were wiped out, except for five pregnant women, who hid in a cave and subsequently gave birth to five boys whom they later married and who then set about repopulating Ireland.
The Welsh forces fared little better, for only seven of the expedition survived, Bendigeid Vran himself suffering a mortal wound. The seven survivors, named as Pryderi, Manawydan, Glifieu, Taliesin, Ynawc (Ynawag), Grudyen (Gruddieu) and Heilyn, carried the severed head of Bendigeid Vran back to Britain and, after a somewhat delayed journey, buried it under the White Mount in London.
Branwen, the cause of the expedition, was brought back to Wales, but died of a broken heart when she sat down to contemplate the destruction brought about on her behalf.