They are obtained by mindreading the witch Ursula at the Forbidden Archive in the Prancing Seahorse on Lanilor Lane. If she is set free, they are automatically added to your inventory. If you choose to fight her, they can be looted from her remains afterwards.
"Blessed by the Maxos rituals, Dragon Knights became natural beacons of magic: the powers of which – be it for creation or destruction, righteousness or mischief – they could use at will and whim. Soon, some of them not only let themselves be engulfed by the sources of sorcery, but wrapped them around their bodies: solidly embedded in blade and cuirass."