Fergus mac Róich

Fergus mac Róich is King of Ulster before Conchobar mac Nessa, losing the throne through Ness's maneuvering on her son's behalf — Fergus grants Conchobar the kingship for a single year as a condition of marrying Ness, and by the year's end the Ulstermen prefer the young king's rule enough that Fergus never regains the throne, remaining at court afterward as one of its most powerful warriors rather than as king. His decisive break with Ulster comes later, over Deirdre and Naoise. Fergus personally guarantees Conchobar's promise of safe passage and forgiveness to lure the exiled Naoise and Deirdre back to Ulster, staking his own honor on the king's word — and Conchobar betrays that guarantee, having Naoise killed regardless. Bound by honor to answer a betrayal made in his name, Fergus kills members of Conchobar's household in retaliation and leaves Ulster for good, taking a large following of warriors with him into exile at the court of Queen Medb of Connacht. This is precisely the position Fergus holds during the *Táin Bó Cúailnge* — fighting on Medb's side against his own former kingdom, and against Cú Chulainn, a warrior he once helped train. His knowledge of Ulster's terrain and tactics from the inside makes him a valuable guide for the invasion, though his old loyalty is never entirely gone; at points in the *Táin* he deliberately withholds his full strength against Cú Chulainn, honoring their earlier bond even as circumstance puts them on opposite sides.