Domain development help?

[Baron Von Stanton]

Generally, the Mists of Ravenloft only claim mortals or undead from the Prime Material Plane (with the noted exception of Vecna being punted into Ravenloft for pissing off the Lady Of Pain inside of Sigil). What did the angel do to attract the Dark Power's attention?

[Mephisto of the FoS]

Generally, the Mists of Ravenloft only claim mortals or undead from the Prime Material Plane (with the noted exception of Vecna being punted into Ravenloft for pissing off the Lady Of Pain inside of Sigil). What did the angel do to attract the Dark Power's attention?

I agree since your villain is an outsider, the fiend or angel has already a reality wrinkle that may be affected by his fungal disease.

[Gonzoron of the FoS]

I would start with story before rules, and ask "what did this darklord do to catch the attention of the dark powers." You've got a very cool aesthetic idea, but how did this angel fall? Why parasites and disease as the visible signs of his fall? Find the act of ultimate darkness that explains that, and you're on your way.