"Several of the Carlist pretenders to the Spanish throne were also known as Don Carlos.
Carlos, Prince of Asturias, also known as Don Carlos (8 July 1545 – 24 July 1568), was the eldest son and heir-apparent of King Philip II of Spain. His mother was Maria Manuela of Portugal, daughter of John III of Portugal. Carlos was mentally unstable and was imprisoned by his father in early 1568, dying after half a year of solitary confinement. His fate was a theme in Spain's Black Legend, and inspired a play by Friedrich Schiller and an opera by Giuseppe Verdi." - (en.wikipedia.org 10.08.2021)