Magister Winonin:
It seems that the most evil creatures in the Devastated Castle are the Doom Servants, Doom Guards, Doom Archers, Doom Troopers, Doom Warriors, and Doom Knights.
Remember that you cannot fill your crystal with the souls of weaker creatures. The crystal develops a refined taste and demands souls of greater evil. If you notice that your crystal has stopped absorbing souls, seek out creatures of greater evil.