The grey talon assassins are a very special group for one reason. They have never been caught. They are impossibly hard to find; many doubt their very existence. Once one has found them, they can be sure the job will be done… for the right price of course.
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.