Sebastian I of Portugal was born on January 20, 1554, and became king at a young age after the death of his grandfather, John III of Portugal. His reign began in 1557 and lasted until his mysterious disappearance in 1578 during the Battle of Alcácer Quibir in Morocco. Sebastian was known for his ambitious plans to expand Portuguese territories. Sebastian's disappearance led to a succession crisis, as he left no heirs. This event contributed to the eventual union of the Portuguese and Spanish crowns under Philip II of Spain in 1580, marking a significant shift in the history of Portugal.