Complex sentence: Upon his death the throne passed to his brother Ceolwulf, because his son Cynegils was presumably too young to inherit the throne, so it was given to the brother, as was probably the custom among the Saxons.
Split into simpler sentences: Upon his death the throne passed to his brother Ceolwulf. Because his son Cynegils was presumably too young to inherit the throne, it was given to the brother, as was probably the custom among the Saxons.