Portuguese royalty, the only son of Prince John and his cousin, the Spanish Princess Joan of Austria, daughter of Charles V. When his mother left for Spain a few months after his birth, Sebastian was left in the care of his grandfather, King John III. He was educated by priests who ingrained deep bigotry and Puritanism and by the old military standards of fanatic chivalry. Historians wrote that he was precocious, strongly built and excelled in hunting and the art of war.
From the age of 11, he was given a regime of purging and was suspected of having diabetes. Sebastian came to the throne at 14. Something of a recluse, he became a self-centered despot. The anti-luxury laws that he issued were obeyed by few of the nobility, who preferred their lifestyle of ostentation hedonism. Sebastian was a misogynist who refused all the brides who were offered to him, but instead, obsessed with a mad idea of military glory in North Africa, he led his army and many of the Portuguese nobility in battles of conquest. On 8/04/1578 (August 14 NS), he led his troops in the battle of Alcacer Kibir, Morocco where he, age 24, and his army were defeated and killed. This precipitated the downfall of Portugal when two years later, Felipe II of Spain seized the throne.
Link to Wikipedia biography