Balor is described as a giant with an eye which wreaks destruction when opened. The Cath Maige Tuired calls it a destructive and poisonous eye that when opened, permits an entire army to be overwhelmed by a few warriors. It was said that four warriors had to lift the eyelid, which became poisonous after Balor looked into a potion being concocted by his father’s druids. Later folklore says that he has only one eye and describes it as follows: “He had a single eye in his forehead, a venomous fiery eye. There were always seven coverings over this eye. One by one Balar removed the coverings. With the first covering the bracken began to wither, with the second the grass became copper-coloured, with the third the woods and timber began to heat, with the fourth smoke came from the trees, with the fifth everything grew red, with the sixth it sparked. With the seventh they were all set on fire, and the whole countryside was ablaze!”.
Source: Balor – Wikipedia