The Krypteia

Plutarch, Life of Lycurgus of Sparta, 28:

"Now in all this there is no trace of injustice or arrogance, which some attribute to the laws of Lycurgus, declaring them efficacious in producing valor (andreia), but defective in producing righteousness (dikaiosyne). The so-called KRYPTEIA at [Sparta], if it really was one of Lycurgus' institutions, as Aristotle says it was, may have given Plato (Laws 630.d) also this opinion of the man and his constitution.

This is as follows: The magistrates from time to time sent out into the countryside at large the most discreet of the young men, equipped only with daggers and necessary supplies. During the day they scattered into obscure and out of the way places, where they hid themselves and lay quiet. But in the night, they came down to the roads and killed every Helot whom they caught. Often, too, they actually made their way across fields where the Helots were working and killed the sturdiest and best of them. So, too, Thucydides, in his History of the Peloponnesian War [IV.80], states that the Helots who had been judged by the Spartans to be superior in bravery, set wreathes upon their heads in token of their emancipation, and visited the temples of the gods in procession, but in a little while afterwards all disappeared, more than two thousand of them, in such a way that no man was able to say, either then or afterwards, how they came to their deaths. And Aristotle in particular says also that the Ephors, as soon as they came into office, made formal declaration of war upon the Helots, so that there might be no impiety in slaying them."