Brenin Llwyd
W.: "Grey King"
"The Monarch of the Mists" in Welsh folklore. The Grey King is said to haunt whatever the local mountain range is, hiding in the mists of the mountain top. Most cited is his association with Snowdonia, particularly Cader Idris or Plinlimmon.
The description of his disposition is decidedly gruesome--he is not only a brooding, silent figure, but lies in wait for unwary travelers who venture up into his mountains. Those who were never seen again were said to have been taken by the Brenin Llwyd. In this, he is a typical earth spirit, representative of the capricious nature of, well, Nature. However, he is not identified with any specific group of Welsh faeries, but is an independent figure.
In his connection to Snowdonia, he may be a confusion with the figure of the giant Idris, ruler of Cader Idris; or, perhapse they are the same spirit. Moreover, the Brenin Llwyd is not the only figure who is said to haunt the mountain tops; Gwyn ap Nudd, ruler of the Tylwyth Teg, is also said to keep to the mountains and hills, particularly Glastonbury Tor. Like Herne the Hunter, Gwyn is also a leader of the Wild Hunt, which is similar to the Brenin Llwyd's hunt of lost hikers, though with a decidedly more moralistic tinge.
The fourth book in Susan Cooper's series The Dark is Rising is named for the Brenin Llwyd, who, when awoken by Will and Bran with the golden harp, leads the Riders against the Dark. The book and series often pulls from Welsh myth and folklore, and in this particular book achieves a rather dark, moody feel, appropriate for the subject matter and the titular figure. The Grey King won the 1976 Newbery Medal.
SOURCES:
Cooper, Susan. The Grey King. NY: McElderry / Atheneum. 1975.
Trevelyan, Marie. "Water-Horses and Spirits of the Mists." Folk-lore and folk-stories of Wales. 1909. OOP, available online at http://www.red4.co.uk/ebooks/trevfolklore.htm.