While the dark elves were working, and after Luke finished learning a new spell, Luke called his friends and told them about his thoughts. The dark elves that he had exterminated probably had been created through the red crystal and considering the direction they came from, it also would be weird if they saw his group near the crater. Shalia and Sylah didn't look that troubled by the fact that their people had been forced into slavery and then sacrificed by some overlord, though. Instead of that, they just helped Luke find some monsters and tame them before sacrificing them to the red crystal.
"Considering how much mana we have now, and we can recover over time, it is only natural that some overlords would try to use us that way," Shalia said. "At least we learned which direction the enemy was in."