The son of Penardun and Eurosswyd, brother to Nisien and half-brother of Bendigeid Vran.
While Matholwch, King of Ireland, was in Wales seeking the hand of Branwen, the daughter of Llyr and sister to Bendigeid Vran, Efnisien cruelly disfigured the hundreds of horses Matholwch had brought with him, an insult so bad that Matholwch took his revenge on Branwen by treating her cruelly after they had returned to Ireland. When Bendigeid Vran learnt of the suffering of his sister, he set out with his host to rescue her.
The Irish at first attempted to placate Bendigeid Vran by housing him in a splendid palace, but they had hidden one hundred warriors inside bags of provisions. Efnisien suspected the trick and circumvented it by crushing the heads of each of the hidden warriors while they were still in hiding. Matholwch and Bendigeid Vran met and decided that the best way to settle their argument was to bestow the kingship of Ireland on Gwern, the boy who had been born to Branwen and Matholwch. On hearing this, Efnisien cast the unfortunate boy into the fire and war broke out.
Because of the cauldron Bendigeid Vran had given Matholwch as a wedding gift, the Irish at first had the upper hand for they could restore their dead to life. Efnisien changed the fortunes of the Welsh when he managed to destroy the cauldron, but he was killed in the attempt. So absolute was the carnage that only five pregnant Irish women remained hidden in a cave, and only seven of the invaders returned across the Irish Sea.