Carlos, Prince of Asturias

Carlos, Prince of Asturias, also known as Don Carlos (8 July 1545 – 24 July 1568), was the oldest son of King Philip II of Spain. He was heir apparent to the throne. His mother was Maria Manuela of Portugal. She was the daughter of John III of Portugal. 
Carlos went mad and so his father put him in prison in early 1568. Carlos died after half a year of solitary confinement. His sad fate became a theme for a play by Friedrich Schiller and an opera by Giuseppe Verdi.