Efnysien was born to Penarddun, the daughter of Beli son of Mynogan, and her second husband Euroswydd. His twin brother was Nisien, a young man of such gentle nature that he could make peace between two hosts even when they were at their most incensed. Efnysien was the opposite in every way. Where Nisien brought harmony, Efnysien would provoke conflict between two brothers even while they were at their most amicable. The Welsh Triads, ancient mnemonic poems that catalogue the great figures of Welsh tradition, record that Llyr, the father of Bendigeidfran, Branwen, and Manawydan, was once one of the Three Exalted Prisoners of Britain, held captive by Euroswydd. This lost tradition hints at a dark origin for Efnysien and Nisien, born as they were from a union forged in captivity and division.

The defining catastrophe of Efnysien's life began on an afternoon at Harlech, where Bendigeidfran sat with his court above the ocean. Thirteen ships approached from Ireland, bearing Matholwch, the king of that island, who sought Branwen's hand in marriage. Bendigeidfran agreed, seeing in the match a chance for lasting peace between Britain and Ireland. A great feast was held at Aberffraw, with Bendigeidfran seated between Manawydan and Matholwch, and Branwen beside the Irish king. They feasted in pavilions because Bendigeidfran was a giant who had never been contained within a house. The marriage was sealed, and Matholwch slept with Branwen that night.

Then Efnysien arrived. He came across the billets where Matholwch's horses were stabled and asked whose they were. When told they belonged to the king of Ireland, who had just married Branwen without Efnysien's consent, something in him snapped. He began striking the horses with a blade, slicing their lips back to their teeth, their ears back to their heads, and their tails to their backs. Wherever he could grip their eyelids, he cut them back to the bone. The animals were ruined beyond use, and the insult was so profound that Matholwch, when he learned what had been done to his horses, prepared to leave at once. Bendigeidfran pacified him with new horses, a gold plate, a silver rod, and finally the Pair Dadeni, a magical cauldron that could bring the dead back to life, though those it restored would be deaf and mute. Matholwch accepted the gifts and took Branwen to Ireland, but the wound of the mutilation festered in his heart.

In Ireland, Branwen gave birth to a son named Gwern, but the memory of the horses proved stronger than the child. The Irish nobles persuaded Matholwch that he had been insufficiently compensated for the humiliation, and Branwen was banished to the kitchens of the castle, where she slept and worked each day and was beaten by the bloody hands of the butcher. She trained a starling, tied a letter to its leg, and sent it across the Irish Sea to Wales. When Bendigeidfran read his sister's message, he gathered an army and marched on Ireland. The giant king was so tall that he waded across the sea while his ships sailed by his shoulder. When the Irish destroyed a bridge to stop him, Bendigeidfran lay across the river himself and said, "He that would be a chief, let him be a bridge." His men walked over him to the other side.

Matholwch, seeing that he could not defeat the giant by force, tried treachery. He built a great house with a hundred pillars to host a feast of reconciliation, and in each pillar he hid an armed warrior inside a bag of flour. Efnysien, suspicious as ever, discovered the plot. He went from bag to bag, crushing the skulls of the hidden Irishmen with his bare hands, killing two hundred men before the feast even began. The peace offering was ruined, but Matholwch tried once more. He agreed to make Gwern, Branwen's son, the king of Ireland, and a coronation feast was held. At the height of the celebration, Efnysien seized the boy and cast him headlong into the fire. Gwern burned, and any hope of peace burned with him.

War erupted. The Irish, armed with the Pair Dadeni, had the upper hand because every soldier who fell was thrown into the cauldron and rose again, deaf and mute but ready to fight. The British army was being ground down until Efnysien saw what must be done. He hid himself beneath the bodies of dead Irish warriors and allowed himself to be thrown into the cauldron. Once inside, he stretched out his living body against the sides of the vessel and pushed with all his strength. The cauldron shattered into four pieces, and Efnysien's heart burst with the strain. He died, but the cauldron was destroyed, and the Irish could no longer revive their dead. The British won the battle, but the cost was absolute. Only seven men survived from Bendigeidfran's army, including Manawydan and Pryderi. Bendigeidfran himself was mortally wounded by a poisoned spear in his foot. Branwen died of a broken heart on the shore of the sea that had carried the message of her suffering. And Efnysien, the man who had caused it all, became the man who ended it, dying inside the very cauldron he had helped put into Irish hands.