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 (Herod.vi. 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.
Demaratus in Wikipedia
Demaratus (Greek: Δημάρατος) was a king of Sparta from 515 until 491 BC, of the Eurypontid line, successor to his father Ariston. As king, he is known chiefly for his opposition to the other, co-ruling Spartan king, Cleomenes I.
When Cleomenes attempted to make Isagoras tyrant in Athens, Demaratus tried unsuccessfully to frustrate his plans. In 501 BC, Aegina was one of the states which gave the symbols of submission (earth and water) to Persia. Athens at once appealed to Sparta to punish this act of medism, and Cleomenes I crossed over to the island to arrest those responsible. His first attempt was unsuccessful, due to interference from Demaratus, who did his utmost to bring Cleomenes into disfavour at home.
In retaliation, Cleomenes urged Leotychidas, a relative and personal enemy of Demaratus, to claim the throne on the ground that the latter was not really the son of Ariston, but of Agetus, his mother's first husband. Cleomenes bribed the Delphic oracle, to pronounce in favor of Leotychidas, who became king in 491 BC.
After the deposition of Demaratus, Cleomenes visited the island of Aegina for a second time, accompanied by his new colleague Leotychides, seized ten of the leading citizens and deposited them at Athens as hostages.
On his abdication, Demaratus was forced to flee. He went to the court of the Persian king Darius I, who gave him the cities of Pergamum, Teuthrania and Halisarna, where his descendants still ruled at the beginning of the 4th century.
He accompanied Xerxes I on his invasion of Greece in 480 BC and is alleged to have warned Xerxes not to underestimate the Spartans before the Battle of Thermopylae:
" The same goes for the Spartans. One-against-one, they are as good as anyone in the world. But when they fight in a body, they are the best of all. For though they are free men, they are not entirely free. They accept Law as their master. And they respect this master more than your subjects respect you. Whatever he commands, they do. And his command never changes: It forbids them to flee in battle, whatever the number of their foes. He requires them to stand firm -- to conquer or die. O king, if I seem to speak foolishly, I am content from this time forward to remain silent. I only spoke now because you commanded me to. I do hope that everything turns out according to your wishes. "
In The 300 Spartans, Demaratus appears as a guest of Xerxes I of Persia, whose warnings about the Spartans Xerxes ignores. Demaratus was played by Ivan Triesault.