Complex sentence: At the time of his death William had no surviving legitimate children, but he was survived by eight of the ten illegitimate children he had by the actress Dorothea Jordan, with whom he cohabited for twenty years.
Split into simpler sentences: At the time of his death William had no surviving legitimate children, but he was survived by eight of the ten illegitimate children. That he had by the actress Dorothea Jordan, with whom he cohabited for twenty years.