Frank_Yarbrough
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I think that this is Dawn/Derius. We know that the horsemen are more or less working together at this point, and Dawn also has a pretty big grudge on Lith. Acala died in the cave and Dawn ran back home in just her crystal form. So it is highly likely she made a deal with Derius to get revenge on both Lith and the Ernas family. This would explain the heat rays and the archmage robes.