King Sebastian of Portugal was born on January 20, 1554, and became king at a young age after the death of his grandfather, King John III. He ruled from 1557 until his disappearance in 1578 during the Battle of Alcácer Quibir in Morocco, where he sought to expand his kingdom. His reign was marked by a desire for military glory and the pursuit of a crusade against Islam. Sebastian's mysterious disappearance led to a crisis in the Portuguese monarchy, as he left no heirs. This event contributed to the eventual union of the crowns of Portugal and Spain under Philip II of Spain in 1580, significantly impacting the history of both nations.