Complex sentence: The two are lovers for many years but their relationship dissolves once Uhtred enters a relationship with Eadith who was the mistress of her late husband, although Æthelflaed and Uhtred remain friends after this and Uhtred does not break his oath to her.
Split into simpler sentences: The two are lovers for many years, but their relationship dissolves once Uhtred enters a relationship with Eadith who was the mistress of her late husband. Regardless, Æthelflaed and Uhtred remain friends after this, and Uhtred does not break his oath to her.