Peter III of Portugal

Born 5 July 1717, Dom Peter III spent most of his life as a royal without a throne until an unusual marriage changed everything. On 24 February 1777, he became King of Portugal - not through inheritance or conquest, but by marrying his niece, Queen Dona Maria I. The union made him king consort, and he ruled alongside her for nearly a decade. Somewhere in those years he earned the nickname "the Builder," though the surviving record is silent on what he actually built. He died 25 May 1786, having spent his final years as the only Portuguese king to have reached the throne by marrying into it rather than being born to it.