The Twelve Mages
The Twelve Mages are the group King Rhobar II assembled to raise the Barrier over the Valley of Mines. They are the reason everything inside the Barrier, and everyone in it, is sealed in.
Composition
The twelve were split evenly between the two magical circles of the Kingdom:
Six Fire Mages from the Circle of Fire, representing the Crown-aligned half of the mage corps. The Fire Mages who live in the Old Camp were part of this group.
Six Water Mages from the Circle of Water, whose leader Saturas later broke away with the New Camp.
The Ritual
The ritual combined all twelve mages' magical strength through a pattern of five magical focuses. The goal was a controlled, localised dome to lock the colony in. What actually happened was that the Sleeper, disturbed deep beneath the valley, bent the weave of the spell. The resulting Barrier was far larger and far more unstable than planned, and the mages lost control of it at the moment of its creation.
The Thirteenth
One former member of the Fire Circle was not among the twelve at the moment of the ritual. Xardas had already left the circle to study necromancy, and by the time the ritual was cast he was working on his own path. He is the single mage from that era who arguably understands what actually went wrong.