The spirits appear to be imbued with the magic of the volcano - indistinguishable from the flow now being held back by the mirror. They are willing sacrifices from the past but not willing to be destroyed by the volcano, as previously believed, but to be a vessel for some of the magic within it. In doing so, they were able to relieve some of the pressure and prevent an eruption but died in the process. Eventually the magic could no longer be contained within their bodies and returned to the core of the volcano.