Storing Raw Magical Energy

The Nethen people had a fascination with magic and all things magical. Their entire society was based around the study of magic. As the science of magic progressed, the desire to store raw magical energy to be used later became of greater and greater interest. While wizards were able to store limited amount in magic items, the desire to create a battery, that would recharge on its own, was needed to create more of the Nethen's ambitious constructs.
At one point, the wizards of a now missing city were able to create a working prototype. As a group, they left to find places to setup their batteries where they could charge quickly. While gone, their battery filled over capacity, and without a method to discharge excess energy, it overcharged and exploded leaving the Crater of Aeoc-Dur.
They were finally successful with their battery design resulting in what is now known as the Karenthian Batteries. They last for a very long time, but eventually are no longer able to hold a charge. The exact time that a battery will live is unknown, as some that were made have already discharged, while the one in Ironholm still operates at full power. It is possible that there are various iterations in the designs allowing for some to last longer than others.
While the wizards worked to develop smaller and smaller versions of the batteries, they were never successful in making a portable version. The batteries require to much shielding and resources to hold the charge of magic, and thus, end up being very heavy.
With the Nethens leaving this part of Karenth, it is not known how the batteries were built, or even how to use them. The Order of Eight have discovered some uses of the batteries, but they are mostly from the resulting side effects caused by the storing of magic.
To this day, no one in the known world knows much about the batteries, or if more exist. Few people in the known world are even aware of the existence of the batteries, and the Order plans to keep it that way.

Complexity

The Karenthian Batteries are extremely complex to build. They require large metallic spheres made of special alloys in order for them to be successful.