Demarātus in Harpers Dictionary of Classical Antiquities

The son and successor of Ariston on the throne of Sparta, B.C. 516. He was deposed, through the intrigues of Cleomenes, his colleague, on the ground of his being illegitimate. After his deposition he was chosen and held the office of magistrate; but being insultingly derided on one occasion by Leotychides, who had been appointed king in his stead, he crossed over into Asia to Darius, who received him honourably and presented him with lands and cities ( 65Herod., 70). He enabled Xerxes subsequently to obtain the nomination to the empire, in preference to his elder brother Artabazarnes, by suggesting to him an argument, the justice of which was acknowledged by Darius (Herod.vii. 3). We find him after this, though an exile from his country, yet sending the first intelligence to Sparta of the designs of Xerxes against Greece. He accompanied that monarch on his expedition, frankly praised the discipline of the Greeks, and especially that of the Spartans; and before the battle of Thermopylae explained to him some of the warlike customs of the last-mentioned people. We learn also that he advised Xerxes to seize, with his fleet, the island of Cythera, off the coast of Laconia, from which he might continually ravage the shores of that country. The monarch did not adopt his suggestion, but still always regarded the exile Spartan as a friend, and treated him accordingly.

Read More about Demarātus in Harpers Dictionary of Classical Antiquities