Balor is the Irish name of a character from Celtic mythology also called King of Fomorians. He belonged to the race of gods called Fomorians.
He had one eye on his forehead and another evil eye on the back of his skull that he usually kept closed. When he opened it, his gaze was deadly to whoever he fixed it on. That look was lightning. Balar was the one who killed the king of the Tuatha Dé Dannan, Nuada, during the Second Battle of Magh Tuiredh. Lug, wanting to avenge Nuada, approached Balar (whose evil eye had closed again after killing Nuada). Realizing that Lug was approaching him he tried to open it again. But Lug was faster, he threw a stone at him with his sling that hit him in the evil eye, piercing his skull. Balar fell dead immediately. Balar was Lug's maternal grandfather, despite which he died at his hands.