Not sure what factors we should put on the hellfire, but I do think there's some potential on the symbols idea. Both symbol of pain and symbol of madness are in d20 Modern as incantations (albeit renamed Sigil of Algos and Sigil of Lyssa), so they have precedence in the game.
Cleon, do you mean to add a disease/curse to the Hellfire or the Foul Miasma? I think I'd prefer the latter.
Alright, how's this sound? Too powerful, or not powerful enough?
"Foul Miasma (Su): The tainted aura of Chernobog brings ill omen and despair to those that fall into its presence. Any creature within 30(?) feet of Chernobog faces being stricken with a supernatural disease that rots the body and poisons the mind (Fort save DC 34 negates; incubation period 1 day; primary 1d8 Constitution, secondary 1d8 Wisdom). A remove disease spell or a Treat Injury check (DC 20) can cure the victim."
Okay, revising it as such, how's this?
"Foul Miasma (Su): The tainted aura of Chernobog brings ill omen and despair to those that fall into its presence. Any creature within 30 feet of Chernobog faces being stricken with a supernatural disease that rots the body and poisons the mind (Fort save DC 34 negates; incubation period 1 day; primary 1d8 Constitution, secondary 1d8 Wisdom). A remove curse spell followed by a remove disease spell can cure the victim. A Knowledge (Arcane Lore) check (DC 30) can also uncover methods of treating the toxin through medical means, allowing the character to make a Treat Injury check at the same DC."