Hafgan

In Welsh mythology, Hafgan was a rival of Arawn's for the position of the god of the underworld. He could not be killed by a god, only a mortal. Arawn switched positions with Lord Pwyll, who then killed Hafgan.

Also in Stephen R. Lawhead's books Taliesin and Merlin Hafgan is the Head Druid, who is on The Side of the Light, He tutors Taliesin, Merlin's father, and in later years Merlin himself.