Leonidas, Λεωνίδας; "Lion's son", "Lion-like") was a king of Sparta, the 17th of the Agiad line, one of the sons of King Anaxandridas II of Sparta, who was believed in mythology to be a descendant of Heracles, possessing much of the latter's strength and bravery. While it has been established that King Leonidas of Sparta died at the Battle of Thermopylae in August, 480 BCE, very little is known about the year of his birth, or for that matter, his formative years. Paul Cartledge has narrowed the date of the birth of King Leonidas to around 540 BCE.
Leonidas was one of three brothers: he had an older brother Dorieus and a younger brother Cleombrotus, who ruled as regent for a while on Leonidas' death before the regency was taken over by Pausanias, who was Cleombrotus' son. Leonidas succeeded his half-brother Cleomenes I, probably in 489 or 488 BCE, and was married to Cleomenes' daughter, Gorgo. His name was raised to heroic status as a result of the events in the Battle of Thermopylae, one of the most famous battles in ancient history.
Jacques Louis David Painting & Text:wikipedia.com