Azorian ward-crystals

Great veins of crystal run underneath the island of Azoria, the wizard kingdom located in the north-east of the Sundered Lands. Ley-lines form rivers of magic in which the crystals grow, and as a result the minerals of the crystals are intensely reactive to magic, and can store it in great quantities.
Specially trained, qualified and bonded miners excavate carefully regulated quantities of the crystal each year. The chunks of crystalline ore are carved into carefully calculated geometric prisms and etched with runes, before being immersed in the thaumaturgical collection pools which gather and focus arcane energies. The resulting crystals glow with power which can be drawn upon by trained mages like a battery.
Attempting to export ward-crystals from Azoria is punishable by death - usually summary execution, if a smuggler is caught in the act by an Azorian Warden. The only crystals which are permitted beyond the borders are granted to agents or Wardens by the Azorian High Council, for specific missions on behalf of Azoria. Even then, the crystals are disguised as everyday items and bound to the life force of the bearer; should the crystal be taken beyond arms' reach of the individual to whom it is bonded, or the connection cut (such as by the bearer's death), the crystal will release all its stored energy in a cataclysmic blast. If there is insufficient energy to destroy it, the crystal will sublimate into vapour instead.
The great advantage of the Azorian defenders is the ward-crystals, great crystalline batteries buried deep underneath the fortifications and educational institutions of the island. Mages sworn to the defence of the island are linked to the ward-crystals, able to feed unused arcane energies to the crystals at the end of each day - and draw upon them in emergencies. The crystals themselves have a limited form of sentience, and power the defensive wards which surround the island. Attackers will find themselves sailing back out of a fog bank with no memory of what transpired within beyond a vague sense of dread, or wander in circles until they starve, or be cut down by blasts of magic seemingly out of the sky.