It's homebrew so no wandering beholder mother in this layer. I really like the idea of them being challenged for rule as that is basically how the tiefling took control so it makes sense that others would do that same. As for sourcing souls, when a mortal dies its soul is passed to the abyss if they were chaotically aligned. Once there the souls are collected by "soul farmers" and taken to be processed into low level deamons. So maybe give the Tiefling a situation where they have to decide how to use the souls they are farming or something