The Spartan educational system included a secret training known as the krypteia or cryptia. The following is from a public domain translation of Plutarch's Life of Lycurgus.Hitherto I, for my part, see no sign of injustice or want of equity in the laws of Lycurgus, though some who admit them to be well contrived to make good soldiers, pronounce them defective in point of justice. The Cryptia, perhaps (if it were one of Lycurgus's ordinances, as Aristotle says it was), Gave both him and Plato, too, this opinion alike of the lawgiver and his government. By this ordinance, the magistrates dispatched privately some of the ablest of the young men into the country, from time to time, armed only with their daggers, and taking a little necessary provision with them; in the daytime, they hid themselves in out-of-the-way places, and there lay close, but, in the night, issued out into the highways, and killed all the Helots they could light upon; sometimes they set upon them by day, as they were at work in the fields, and murdered them. As, also, Thucydides, in his history of the Peloponnesian war, tells us, that a good number of them, after being singled out for their bravery by the Spartans, garlanded, as enfranchised persons, and led about to all the temples in token of honors, shortly after disappeared all of a sudden, being about the number of two thousand; and no man either then or since could give an account how they came by their deaths. And Aristotle, in particular, adds, that the ephori, so soon as they were entered into their office, used to declare war against them, that they might be massacred without a breach of religion.
~ Plutarch's Life of Lycurgus