User blog comment:Lex wa Kaiho/What is Lumen Histoire?/@comment-7418318-20141020100143

Thanks to the title of the next chapter "The girl in the crystal" I have a new theory and I can't imagine for the world why I didn't think of this before:

Lumen Histoire is a Crytal (maybe somehow related to the Tenrou Jade?) that has encased Mavis' body a moment right before her death. And the Mavis we saw isn't her ghost, but a psychic projection.