Magister Gideon:
I see that you have 10 souls in your Soul Crystal. Usually that crystal would be considered full.
However, there is a way to increase that limit. I only disclose this method to those who I feel will use it prudently. You seem reasonably competent, so I'll tell you how to do it.
The Soul Crystal that absorbed 10 souls refuses to absorb souls of ordinary evil spirits any more. Yes. You must seek out and destroy creatures that are more evil than most, so evil in fact that there are only a few of them in this world.
Among the beasts currently existing in this continent, you must slay at least one of Baium, Zaken, Anakim and Lilith. Then you can upgrade your Grade 10 Soul Crystal to Grade 11 and to Grade 12.
However, to upgrade the Grade 12 Soul Crystal to Grade 13, you must slay one of the beasts with more powerful souls, which are Valakas, Antharas, Amber and Messenger Anakazel, and absorb its soul. However, it is much harder to successfully absorb souls of Amber and Messenger Anakazel than Valakas or Antharas.
For your reference...
For your reference?