Complex sentence: Agnes of Essex, countess of Oxford ("circa " 1151 -"circa" 1206) was the daughter of Henry of Essex and his second wife, she was betrothed at age three to Geoffrey de Vere, brother of the first earl of Oxford.
Split into simpler sentences: Agnes of Essex, countess of Oxford ("circa " 1151 -"circa" 1206) was the daughter of Henry of Essex and his second wife. She was betrothed at age three to Geoffrey de Vere, brother of the first earl of Oxford.