"They're many names given to the things of this vast consciousness that we don't understand, are afraid of, or otherwise just don't want to be associated with. These creatures, though familiar in appearance to many common races, have no name for themselves, but have been appropriately named "Concordare Iram", Translated: Harmonized Rage."
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.