In Welsh mythology, Efnisien was the son of Penarddun and Eurosswydd. His twin was Nisien.

His half-sister, Branwen, was courted by an Irishman named Matholwch, who gave her brother, Bran horses to curry favor. Efnisien mutilated the horses; Matholwch was irate until Bran gave him a cauldron which restored the dead to life.

Branwen, was treated cruelly by her husband, Matholwch. Bran sailed from Wales to Ireland to rescue her with his brother, Manawydan. When Matholwch saw the giant, he asked for peace and built a house big enough for him. Matholwch agreed to let Bran live with them and give the kingdom to Gwern, his son by Branwen. The Irish lords didn't like the idea, so they hid themselves in flour bags to attack the Welsh. Efnisien guessed what was happening and threw them into the fire, along with Gwern.

In the ensuing war, Efnisien and Bran died. Efnisien threw himself in the cauldron earlier given to Matholwch, which broke; he stayed dead. Branwen went to Aber Alaw and died there. Bran's head, still alive, was buried in England. Legend said that as long as the head was there, England would live on.