Complex sentence: He died peacefully on his summer residence in the island of Mainau (Germany) on September 28 1907,that is now owned by the descendants of his great-grandson Count Lennart Bernardotte (1909-2005).
Split into simpler sentences: He died peacefully on his summer residence in the island of Mainau (Germany) on September 28 1907. Mainau is now owned by the descendants of his great-grandson Count Lennart Bernardotte (1909-2005).