Bran was born the son of Llyr, a deity of the sea whose name connects him to the primal waters that surround Britain, and Penarddun, a daughter of the divine house of Don. His name, Bendigeidfran, means "Blessed Crow" or "Blessed Raven," linking him to the bird of prophecy and the Otherworld that recurs throughout Celtic mythology. He was a giant in the truest sense: no house in Britain could hold him, and he was forced to sit outdoors even in council, his head brushing the clouds while his feet rested upon the earth. His brother was Manawydan, a figure of patience and wisdom who would later marry the widow of King Pryderi, and his half-brother was Efnisien, a man of violent temper whose single act of cruelty would ignite a war that consumed two nations. Bran's sister was Branwen, whose name means "White Raven," the most beautiful woman in Britain and the prize that would be offered to forge peace with Ireland.

The tragedy began with a marriage. Bran gave his sister Branwen to Matholwch, the King of Ireland, to seal an alliance between the two islands. The wedding feast was held at Harlech, and the celebrations were lavish. But Efnisien, Bran's half-brother, was furious that he had not been consulted about the match. In a drunken rage, he mutilated Matholwch's horses, slashing their lips, ears, and tails so that they could never be ridden again. Matholwch, deeply insulted, prepared to return to Ireland in anger. Bran, desperate to salvage the peace, offered lavish compensation: new horses, gold, and above all, the Cauldron of Rebirth, a magical vessel that could restore the dead to life, though those it revived could not speak. Matholwch accepted and returned to Ireland with Branwen and the cauldron, but the seeds of destruction had already been planted.

In Ireland, Branwen gave birth to a son, Gwern, but the memory of the mutilated horses poisoned her life. The Irish court turned against her, and she was banished to the kitchens, where she was beaten daily by the butcher. She trained a starling to carry a letter across the Irish Sea to her brother Bran at Caer Seint in Gwynedd, and when Bran read the message, he gathered the greatest army Britain had ever seen and sailed for Ireland. He was so enormous that he waded across the sea while his men followed in ships, his body a living bridge across the waves. When the Irish retreated across the River Shannon and burned the bridges behind them, Bran lay down across the river and let his army march over him, declaring, "Let him who is a chief be a bridge." The words became a proverb of leadership in Wales, a reminder that the king's body is the property of his people.

The war that followed was a catastrophe for both nations. Matholwch tried to make peace by offering the kingship of Ireland to Gwern, Branwen's son, and building a great hall for the coronation feast. But the Irish plotted treachery: they hung one hundred leather bags in the hall, supposedly filled with flour, but each one concealed an armed warrior. Efnisien, sensing the trap, crushed the skull of each hidden man as he pretended to check the bags for flour. When Gwern was brought forward to be crowned, Efnisien seized the boy and threw him headlong into the fire, killing him instantly and destroying any hope of peace. The battle erupted, and the Irish used the Cauldron of Rebirth to revive their fallen warriors, making them nearly unstoppable. In desperation, Efnisien hid beneath a pile of Irish corpses, was thrown into the cauldron, and broke it apart from within by pushing against its sides with his dying strength. The cauldron shattered, but the cost was total: almost every warrior on both sides was dead.

Bran himself was mortally wounded by a poisoned spear in the foot, a wound that no healer could close. As he lay dying, he commanded his seven surviving companions, including his brother Manawydan and the warrior Pryderi, to cut off his head and carry it back to Britain. They did so, and the head remained alive, speaking with the same wisdom and humour that Bran had possessed in life. The seven men carried it to Gwales in Pembrokeshire, where they sat in a great hall with the head at the table, and for eighty-seven years they feasted and forgot their sorrows, unaware that time was passing. Bran warned them that a door facing Cornwall must never be opened, for as long as it remained shut, his head would protect them. But one day the door was opened, and the spell broke. The men remembered their losses, buried Bran's head at the White Hill in London facing France to ward off invasion, and scattered to their homes. The Triads of the Island of Britain record this as one of the Three Fortunate Concealments, and its undoing as one of the Three Unfortunate Disclosures, for King Arthur later dug up the head, declaring that he alone would protect Britain. The Saxons came soon after.