The metal balls were filled with a substance that was similar to gunpowder, the only difference being that it could interact with mana, simply infusing a little bit of mana in the ball would create a reaction in the substance and it would explode in a violent fashion after a collision.
The metal balls were created by Raigon, of course, he wasn't the only one that had understood the value of the substance they called 'Licin' as there were other kingdoms that used it in a similar fashion. The explosive arrows that the kingdom used for one were something that was created with the use of the substance, the soldiers only needed to infuse the slightest amount of mana in order to activate the licin that was inside of the arrow in order for it to become an explosive one.
