Leonidas I was a hero-king of Sparta

Sparta

Sparta or Lacedaemon, was a prominent city-state in ancient Greece, situated on the banks of the River Eurotas in Laconia, in south-eastern Peloponnese. It emerged as a political entity around the 10th century BC, when the invading Dorians subjugated the local, non-Dorian population. From c...

, the 17th of the Agiad line, one of the sons of King Anaxandridas II

Anaxandridas II

Anaxandridas II was a king of Sparta, son of Leon, between 560 to 525 BC. At the time when Croesus sent his embassy to form alliance with " the mightiest of the Greeks," i. e. about 554, the war with Tegea, which in the late reigns went against them, had now been decided in the Spartans' favour,...

 of Sparta, who was believed in mythology to be a descendant of Heracles

Heracles

Heracles ,born Alcaeus or Alcides , was a divine hero in Greek mythology, the son of Zeus and Alcmene, foster son of Amphitryon and great-grandson of Perseus...

, possessing much of the latter's strength and bravery. Leonidas I is notable for his leadership at the Battle of Thermopylae

Battle of Thermopylae

The Battle of Thermopylae was fought between an alliance of Greek city-states, led by King Leonidas of Sparta, and the Persian Empire of Xerxes I over the course of three days, during the second Persian invasion of Greece. It took place simultaneously with the naval battle at Artemisium, in August...

.
According to Herodotus

Herodotus

Herodotus was an ancient Greek historian who was born in Halicarnassus, Caria and lived in the 5th century BC . He has been called the "Father of History", and was the first historian known to collect his materials systematically, test their accuracy to a certain extent and arrange them in a...

, Leonidas' mother was his father's niece and had been barren for so long that the ephor

Ephor

An ephor was the leader of ancient Sparta and shared power with the Spartan king...

s, the five annually elected administrators of the Spartan constitution, tried to prevail upon King Anaxandridas to set aside his wife and take another.