Slaves of New Arkos
Many ideas abound on the nature or purpose for the hedrons, but here are the two primary schools of thought.
Tiefling Magic: When the Tieflings of Bael Turath destroyed the original city with their powerful magic, they also created the hedrons to prevent any Dragonborn from returning to the site of their most important city. The deadly elemental forces contained within the hedrons would destroy any living thing within their reach. However, when King Maddanock reconquered the abandoned city, he brought with him a powerful artifact recovered from far away lands that could control the power of the hedrons. It was only with this powerful magic, that Maddanock was able to repopulate the city. This power over the hedrons has been passed from reigning monarch to heir so that in times of desperate need, the ruling King or Queen can use the power of the hedrons to blast the enemies of the kingdom to the Abyss.
Dwarven Gifts of Prophesy: The Hedrons were constructed by the Dwarven nation of Gal Morag and gifted to the Arkosian Empire per the terms of a treaty uniting the two nations. The rune covered stones were designed to magically determine the manner and timing of the destruction of the Arkosian Empire, but the proud Dragonborn refused to believe their mighty empire would ever be destroyed. They buried or neglected the hedrons while marching to their destruction. Having failed to prevent the destruction of the city, the powerful magics imbued into the floating stones have begun to slowly unravel, becoming more and more deadly with each passing year. It is imperative that this unraveling be stopped before the city is destroyed again.