Balor is the son of Buarainech and husband of Cethlenn and the father of 12 'white-mouthed' sons, all of them among the chief men of the Fomor. He’s the god of death and the king of the Fomorians, a group of supernatural beings. He is often described as a giant with a large eye in his forehead that wreaks destruction when opened. It was said that anything he looked at would die instantly. The Cath Maige Tuired calls it a "destructive" and "poisonous" eye that no army can withstand, and says that it takes four men to lift the eyelid.
In later folklore it is described as follows: "It was always covered with seven cloaks to keep it cool. He took the cloaks off one by one. At the first, ferns began to wither. At the second, grass began to redden. At the third, wood and trees began to heat up. At the fourth, smoke came out of wood and trees. At the fifth, everything got red hot. At the sixth...... At the seventh, the whole land caught fire".
He has also been interpreted as a god or personification of drought and blight; he also had the power of putting on a different shape.
Balor heard a prophecy that he would be killed by his grandson. To avoid his fate, he locks his only daughter, Ethniu, in a tower on Tory Island to keep her from becoming pregnant. One day, Balor steals a magical cow of abdundance, the Glas Gaibhnenn, from Goibniu the smith. He takes it to his fortress on Tory Island. Cian, who was guarding the cow for Goibniu, sets out to get it back. With the help of the druidess Biróg and the sea god Manannán, Cian enters the tower and finds Ethniu. They have sex, and she gives birth to three sons. Balor attempts to drown the boys in the sea, but one is saved and is raised as a foster-son by Manannán. He grows up to become Lugh.
Lugh eventually becomes king of the Tuatha Dé Danann. He leads the Tuath Dé in the second Battle of Mag Tuired against the Fomorians, who are led by Balor. Ogma disarms Balor during this battle, but Balor kills Nuada with his eye. Lugh kills Balor by casting a sling, or a spear crafted by Gobniu, through his eye. Balor's eye destroys the Fomorian army. Lugh then beheads Balor.
When Balor was slain by Lugh his eye was still open when he fell face first onto 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 Súil, or "Lake of the Eye", in County Sligo.