Complex sentence: Though mentally handicapped, he was also the second of only four of their ten children to survive to adulthood and on the death of his elder brother John in 1537 he succeeded him as hereditary prince of the Duchy of Saxony.
Split into simpler sentences: Though mentally handicapped, he was also the second of only four of their ten children to survive to adulthood. He was mentally handicapped after the death of his elder brother John in 1537 he succeeded him as hereditary prince of the Duchy of Saxony.