Fachtna Fáthach ("the wise"), son of Cas (or Ross), son of
Rudraige, was, according to medieval Irish legend and historical tradition, a
High King of Ireland. He came to power when he defeated the previous High King,
Dui Dallta Dedad, in the battle of Árd Brestine. According to some stories he was the lover of
Ness, daughter of
Eochaid Sálbuide, king of
Ulster, and the father of her son,
Conchobar mac Nessa, the king of Ulster in the stories of the
Ulster Cycle.