a subscriber writes "i was watching the history channel's docupictuary on the spartans last week, and could swear i heard them say the guy who showed the persians the mountain path to get behind the spartans at thermopylae was Ephialtes and that his name became the synonym/eponym for nightmare.
found this on a site about mein kampf"

Herodotus (Book VII, 213-218) tells the story of how a Greek traitor, Ephialtes, helped the Persian invaders at the Battle of Thermopylae (480 B.C.) When the Persian King, Xerxes, had begun to despair of being able to break through the Greek defence, Ephialtes came to him and, on being promised a definite payment, told the King of a pathway over the shoulder of the mountain to the Greek end of the Pass. The bargain being clinched, Ephialtes led a detachment of the Persian troops under General Hydarnes over the mountain pathway. Thus taken in the rear, the Greek defenders, under Leonidas, King of Sparta, had to fight in two opposite directions within the narrow pass. Terrible slaughter ensued and Leonidas fell in the thick of the fighting.

The bravery of Leonidas and the treason of Ephialtes impressed Hitler, as it does almost every schoolboy. The incident is referred to again in Mein Kampf (Chap. VIII, Vol. I), where Hitler compares the German troops that fell in France and Flanders to the Greeks at Thermopylae, the treachery of Ephialtes being suggested as the prototype of the defeatist policy of the German politicians towards the end of the Great War."

http://www.ety.com/HRP/booksonline/mk/mkfootnotes.htm