Magister Gideon:
When a creature's energy is low apply the crystal to him. His soul will be drawn into the crystal. Then, the soul of the evil creature will get pulled towards the Soul Crystal. Afther that if you kill the evil creature, its body and soul will get separated and its soul will be collected into the Soul Crystal. Each crystal can contain up to 10 souls. A more precise explanation would be that it can contain up to 10 souls with regular usage. I will tell you the detailed story later when you bring back the Soul Crystal with 10 souls.
Actually, collecting souls that can be used to bestow special powers to a weapon is not a very easy task.
"What kind of precautions should I take when collecting souls?"