Well, Matt, this NPC is very easy to miss. You know there are a few flying platforms in Atrixus. One of them leads to the traitor guy you need to eventually kill. There is another in the (middle?) of the map where the yellow circle-not-quite-pentagram thingy is, the area with the inn and the demon who had his son captured. If you take this flying platform, you will be taken to a small flying island where another Demonologist Demoness lives. She sells 4 spells in my game (Volcano, Armageddon and 2 weaker ones) + Demons and Demonologists.
Could have easily missed her out myself.
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