Before the Tuatha De Dannen conquered Ireland for their own, the land was ruled by a race known as the Fomorians (or Firbolg). To these beings the Tuatha paid tribute. The Tuatha lords even intermarried with them, although they were a curious race indeed. It was thought that they had once been sea rovers in the oceans off Africa. Whatever their origins, they were a tatterdemalion remnant of the chaotic powers of creation. Some had a single leg and a single eye. Some had men's bodies and the sleek heads of horses. Some looked like fish, scaled and oblong, but walked upright on flipper-like feet, their doeful human faces peering from bullet heads. A very few were beautifully human, at least in form.

The warlike Fomorians dominated Ireland, living by taxes imposed on the bounty of the country's fields and pastures, although they preferred to remain secluded in their stronghold, an island off the northeast coast. The place was an awesome sight from the sea. At its center rose a tower as translucent as glass, and within this tower, according to the chroniclers, could be seen "something that had the appearance of men." Those who ventured too near the island-fortress might find their ship swamped by spell-sent waves.

The chief of the Fomorian tribe was named Balor of the Strong Blows. He was a cyclops, and the single eye he had was kept shut. Its very glance killed. In times of ultimate need during battle, his guides would lift his eyelid with an ivory ring, and all who met that gaze died where they stood, inwardly seared by some nameless power from the dawn of creation.

Though the battle between the Tuatha and the Fomorians was bloody and savage, the fair race of elves won at the last. Aid came in the form of that many formed goddess, the Morrigan, who rained firestorms, a hail of frogs, and torrents of blood that soaked the ground into a bog upon her chosen side's foe. While this ploy of the triple goddess bought time for the Tuatha De Dannen, it was Balor's grandson who mustered the armies of Tuatha and dealt the blow that ultimately won the war.