Very intriguing piece. Traditionally, stories of the fae paint them as confusing being, ones with morals and concerns that are alien to mere mortals. I like what you've done here all in all. It surprised me a little to see Amicus speaking as if Ignis was his familiar instead of the other way around. His possessiveness around Arline is kind of cute, but it does also highlight how the bond they've struck may have different connotations for each of them.
Positioning the hell-flame as something keeping Ignis alive is a nice twist, and a good way to keep her coming back to it. Treating her as half-fae is a direction I don't think I'd have thought of. Lots of potential there, lots of freedom to develop new rules and explore new ideas. Very interesting, also, how Ignis and Deni are going side-by-side down opposite paths, with Ignis veering toward nature and harmony while he heads toward debauchery and sin.