William IV of the United Kingdom

William Henry was George III's third son, a royal spare who went to sea and earned the nickname "Sailor King" during years in the Caribbean and British North America. Created Duke of Clarence in 1789, he spent two decades living openly with actress Dorothea Jordan, fathering ten children outside marriage before she left in 1811. He married Princess Adelaide of Saxe-Meiningen in 1818, served as Lord High Admiral in 1827, and inherited the throne in 1830 when both elder brothers died without legitimate issue. His seven-year reign pushed through the Reform Act of 1832, abolished slavery across nearly all the British Empire, restricted child labour, and updated the Poor Law - though he remained the last British monarch to appoint a prime minister against Parliament's will. He died in 1837 with no surviving legitimate children, splitting his two kingdoms: niece Victoria took Britain, brother Ernest Augustus took Hanover, and the personal union that had lasted 123 years came to an end.