Complex sentence: His son Eärnur, however, became the last King: still during his father's reign, he led the forces of Gondor to the aid of Arthedain in the north and was offended there by the Witch-king of Angmar.
Split into simpler sentences: His son Eärnur, however, became the last King. During his father's reign, he had led the forces of Gondor to the aid of Arthedain in the north and was offended there by the Witch-king of Angmar.