hmmm, really? I thought hell left once the Infeernals were gone. Maybe I'm mis-remembering...
I only wish this was true! Wouldn't it make sense?
I dispatched Hyborem and his small band of Infernals only THREE TURNS after he appeared, but the damage was done and the stuff kept spreading. Despite having 3-4 Adepts assigned fire-fighting duties, it kept popping up allover the map. It was particularly annoying when it appeared to travel invisibly down rivers showing up on tiles at the mouth of a river than ran quite a distance.

I guess because I enjoy playing Good or Neutral civs, I find the hellfire thing the most annoying 'feature' of FFH.
PS. I still cannot understand the dynamic that has Hyborem returning when the counter hits 70 even if he was destroyed a long time ago.
