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.In Stephen R. Lawhead's Pendragon Cycle, a reinterpretation of the King Arthur legend, Hafgan is the Chief Druid of the native Britons. He tutors Taliesin, father of Merlin, and in later years Merlin himself.
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[Celtic] In Welsh mythology, Hafgan battled with Arawn for the dominion of the underworld. When Arawn traded places with Pwyll for a year and a day, Pwyll defeated Hafgan at the end of this period.