King Sebastian was a monarch of Portugal who reigned from 1557 until his disappearance 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 desire for military glory, which ultimately led to his ill-fated expedition against the Moors in the Battle of Alcácer Quibir. Sebastian's mysterious disappearance during the battle left Portugal without a clear successor, leading to a succession crisis. His absence sparked legends and myths, including the belief that he would return as a messianic figure, known as Sebastianism. This cultural phenomenon influenced Portuguese society and politics for years to come.