Fergus mac Róich
In the Ulster Cycle there is the text "Clann Uisnigh" or "Deirdre of the Sorrows" in which Naoise mac Usnech falls for Deirdre, who was imprisoned by King Conchobar mac Nessa due to a prophecy that Ulster would plunge into civil war due to men fighting for her beauty. Conchobar had pledged to marry Deirdre himself in time to avert war, and takes his revenge on Clann Uisnigh. The death of Naoise and his kin leads many Ulstermen to defect to Connacht, including Conchobar's stepfather and trusted ally Fergus mac Róich, eventually precipitating the "Táin Bó Cúailnge".