Magister Gideon:
You probably already know that you can carry up to 10 spirits in the Soul Crystal.
There is, however, a way to absorb more than 10 spirits into the stone! This is a secret we only share with those we consider qualified, and this certainly seems to describe you!
A crystal that has already absorbed 10 souls will reject the spirit of an ordinary devil. But a certain, much more powerful spirit... Well, that's another story!
"A more powerful spirit?"