A common tool of necromancers, nether mages, and others who are privvy to the arts of the dead and undead
A simple way to a fate most dreadful.
A wet brown sack… FULL OF MAGGOTS!
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.