Foster father of Cú Chulainn, knight of the Red Branch and former King of Ulster before being tricked out of his throne by Conchobar mac Nessa. He took Cú Chulainn from his mother Deichtine to serve as a soldier for Ulster. Later after Cú Chulainn returned from the land of shadows, Fergus defected to Ulster's enemy, Connacht, after his sons were killed. Spurring on Queen Medb of Connacht out of hatred for Conchobar, they went to war with Ulster, cornering Conchobar during the decisive battle. 