Magister Winonin:
I see that you have 10 souls in your crystal. Most people would say that that crystal is full, am I right?
But there is a secret method that allows more souls to be absorbed. I only disclose this method to those who I feel will use it responsibly. You seem qualified, so I'll show it to you.
The Soul Crystal that absorbed 10 souls refuses to absorb souls of ordinary evil spirits any more. Yes. You must seek creatures of greater evil than any you have seen before.
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?