Demaratus was the Spartan king deposed and exiled after accusations involving King Cleomenes I.
Demaratus belonged to Sparta’s Eurypontid royal house and reigned alongside the Agiad king Cleomenes I. Their rivalry became intense after Demaratus opposed Cleomenes’ intervention in Aegina during the Persian Wars. Cleomenes then supported a claim that Demaratus was not the legitimate son of his predecessor, Ariston.
The dispute led to an investigation and Demaratus’ removal from the throne around 491 BCE. He fled to the Persian Empire, where King Darius I received him. Demaratus later advised Xerxes I during the invasion of Greece in 480 BCE, including by explaining Spartan military customs.
Leotychidas replaced Demaratus as Eurypontid king. The episode is chiefly known through Herodotus, and its political motives are important: the legal argument about ancestry arose within a broader struggle between Sparta’s two royal houses.