Magister Winonin:
It seems that the most evil creatures in the Lair of Antharas are the Malruk Knights, Malruk Berserkers, Malruk Lords, Limal Karinness, Kariks, Pytans, and Pytan 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.