Balor embodies the concept of the "evil eye" and had an eye in the middle of his forehead and one directly opposite at the back, which made it all but impossible to sneak up on the giant.
According to prophecy, Balor was to be killed by his grandson and like many antagonistic forces in mythology Balor sought to avoid his fate, which he did by locking his daughter, Ethlinn, in a tower made of crystal to keep her from becoming pregnant. However, Cian, one of the Tuatha Dé Danann (traditional enemies of the Fomorians), with the help of the druidess Birog, managed to enter the tower. She gave birth to triplets by him, but Balor threw them into the ocean in an attempt to kill them. 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, which was bestowed with a horrific curse that allowed it to literally fire a beam of deadly magic at will. 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.