Sneaky bastards, the lot of them
Some sovereigns prefer a different sort of tax from their subjects - whether freely given, or taken by force.
A dreamer is one who can only find his way by moonlight, and his punishment is that he sees the dawn before the rest of the world
A rumour of the knights of some powerful king that sleep until danger threatens the kingdom.
The truth is that they were rebels, trying to split the kingdom. Once captured they were put into an enchanted sleep, protected from age and harm, until their aims have been carried out by others in which case they will awaken.
There is a loophole in the spell in that trying to take one of their weapons will also awaken them.