Sebastian I was the King of Portugal from 1557 until his death in 1578. He became king at a young age and was known for his ambitious plans to expand Portuguese territories, particularly in North Africa. His reign was marked by a strong desire for military glory, which ultimately led to his downfall. In 1578, Sebastian led an ill-fated expedition to Morocco, where he was defeated and disappeared during the Battle of Alcácer Quibir. His mysterious disappearance led to a succession crisis, as he left no heirs. This event significantly impacted the future of the Portuguese Empire and contributed to the eventual union with Spain.