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In Irish mythology, Balor of the Evil Eye (sometimes spelled Balar or Bolar) was a king of the Fomorians, a race of giants. His father was Buarainech and his wife was Cethlenn. According to legend, he lived on Tory Island.
Balor was notable for his eye in the middle of his forehead and one directly opposite at the back, which meant he couldn't be sneaked up on from behind. According to prophecy, Balor was to be killed by his grandson. To avoid his fate, he locked his daughter, Ethlinn, in a tower made of crystal to keep her from becoming pregnant. However, Cian, one of the Tuatha Dé Danann, managed to enter the tower with the help of the druidess Birog. Ethlinn gave birth to triplets by him, but Balor threw them into the ocean. Birog saved one, Lugh, and gave him to Manannan mac Lir, who became his foster father. He was called Lugh Lamhfada and became a member of the Tuatha Dé Danann.
Lugh led the Tuatha in the second Battle of Magh Tuiredh against the Fomorians. Ogma disarmed Balor during this battle, but Balor killed Nuada with his eye. Lugh shot a sling-stone, which drove Balor's eye out the back of his head, where it continued to wreak its deadly power on the Fomorian army. In other versions, Lugh blinded Balor with a spear made by Goibniu or decapitated him and used his eye against the Fomorians.
One legend tells that, when Balor was slain by Lugh, Balor's eye was still open when he fell face first into the ground. Thus his deadly eye beam burned a hole into the earth. Long after, the hole filled with water and became a lake which is now known as Loch na Súl, or "Lake of the Eye", which is to be found in County Sligo.
The folklorist Alexander Hagerty Krappe (1894–1947)  discusses the Balor legend in his book Balor With the Evil Eye: Studies in Celtic and French Literature (1927). Krappe believes Balor comes from a very ancient myth, perhaps, he suggests, going back to fertility rites at the time of the introduction of agriculture, of a woman (the earth) shut away by an old man (the old year), impregnated by another man, whose child (the new year), then kills the old man. Other versions of this myth: Gilgamesh, Osiris, Balder, Danaë, Balor in Ireland, the "May Count" in Sweden, and "it has even penetrated to Uganda, where it is told of a local chief." Moreover, according to Krappe, Balor is related to Janus, Kronos, the Serbian monster "Vy," the Welsh Ysbaddaden Pennkawr, and other versions of a two-headed god with an evil eye. Krappe also suggests that the woman may originally have been a cow goddess, such as Hathor, Io or Hera.
See also 
- Journal of American Folklore, Vol. 61, No. 240 (Apr. - Jun., 1948), pp. 201-202
- J.A. MacCulloch, The Childhood of Fiction," London, 1905, p. 411
- W.R.S. Ralston, Russian Folk Tales, London, 1873, p. 72, cited by Krappe, p 4
- The Poetry of the Celtic Races, Ernest Renan. Translated by W. G. Hutchison, II