Grand Magister Jurek:
It seems that the most evil creatures in Antharas' Lair are the malruk knights, malruk berserkers, Malruk lords, Limal karinnesses, 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, try killing creatures of more evil.