Complex sentence: Upon the death of his father on 31 October 1785 he became William IX, Landgrave of Hesse-Kassel and as such, is said to have inherited one of the largest fortunes in Europe at the time.
Split into simpler sentences: Upon the death of his father on 31 October 1785, he became William IX, Landgrave of Hesse-Kassel. He was said to have inherited one of the largest fortunes in Europe at the time.