Ferdiad

In Irish mythology, Ferdiad (also Fer Diad, Ferdia) was the son of Daire (or son of Damáin son of Daire) and the champion of the men of Domnand (Fir Domnann) from Irrus Domnann in Connacht. Along with the Fir Bolg, the Fir Domnann were among the early inhabitants of Ireland before the coming of the Tuatha de Danaan or the Milesians. "Firdiad" means "man/warrior of the pair." ("Diad" related to dïas, or "two persons.")

More famously, Ferdiad was Cúchulainn's best friend and foster-brother. He trained with him for years in Scotland under the famed She-Warrior Scáthach, and they were considered equally extraordinary warriorsâ€"though Cúchulainn had the Gae Bulg, Ferdiad possessed a coat of horn-like armor that no ordinary weapon could pierce. He was persuaded to fight against Cúchulainn by Queen Medb in the Táin Bó Cúailnge as a last resort, Cúchulainn having already slain many other champions in combat. Though Ferdiad initially refused to fight his foster-brother, Medb threatened him with vicious satires and offered him her daughter Findabair if he was successful. Finally and reluctantly, he agreed to go to battle.

After three days of battle at a river ford, Ferdiad was killed when Cúchulainn used his magical spear the Gae Bulg, which once thrown could not be stopped.

Scholars believe that the fight between Cuchulainn and Ferdiad is a late addition to the Táin, originating not earlier than the eleventh century and drawing on earlier episodes in the story.