Welsh: "The Bad One" ef- negative prefix; niessin- "peacemaker" or "good"
Efniessin, son of Penardun and Eurowysedd, brother of Niessin and half-brother to the Children of Llyr--Bran, Branwen, and Manawyddan. Some legends state that he and his brother are the result of a rape of Penardun by the Roman General Ostorius (Welsh--Eurowysedd). In legend, he is a trickster figure, a Loki type of character who constantly sews discord amongst his siblings and other nations. It is his doing that brought about the war with Ireland in "Branwen uerch Llyr," but, in an ironic twist, he is also the one to end the war by destroying the Cauldron of Rebirth, puposfully killing himself in the process as self-punishment for his deeds.