Grand Magister Jurek:
The collection of a soul doesn't always go as planned. Some evil souls will simply not be absorbed, and, if you happen to collect the wrong soul, the crystal will shatter...
Soul crystals develop refined tastes, and will reject weaker souls after a while. When this happens you must seek the souls of creatures of greater evil to satisfy your crystal.
Remember that you may not absorb a soul with more than one crystal because a resonance effect is created between the crystals that prevents proper absorbtion.
Receive a soul crystal.