No , the Greek historian Herodotus (c. 484-425 BC) was not present at the battle of Thermopylae in 480 BC. .
He won.
Xerxes I of Persia was defeated in the naval battle of Salamis by Athenian admiral Themistocles .
Leonidas commanded the Spartan contingent of 300 heavy infantry and 2,100 light infantry at Thermopylae plus another 7,000 from other city-states.
If by spy, you mean the person who betrayed the Spartans, that would be Ephialtes of Trachis.
The Battle of Thermopylae .
The Battle of Thermopylae .
King Xerxes.
Xerxes I.
Xerxes the first
False
King Xerxes I.
Xerxes I of Persia
Ephialtes.
Their king, Xerxes I.
It was a very temporary holdup in his inexorable march south.
Persia - King Xerxes I. Greek allies - King Leonidas of Sparta.
Xerxes I, son of Darius I, was victorius at Thermopylae.