Ouch, I was gone for a while. A few hours after I posted that, I realized it was kind of a dumb idea. Like Good Sauce said, a LOT of work for a relatively minor effect. But the idea occured to me because I really like the OO religion over AV, always was a fan of the Cthulhu mythos.
I was thinking that instead of "Infernal Pact", the tech could be replaced by something like Judgement Day. AV would spawn demons from Hell, and it'll still remain the coolest of the Judgement Day results.
If judgement day is researched by a guy with the state religion of OO, it would eventually drive ALL the world's units insane (not enraged). The good civs and low tech civs who've avoided magic and religion would be less affected by it just the way they're less affected by Hell terrain.
Order could have an apocalypse similar to in Conquest of Elysium where the 4 riders of the apocalypse appear and barbarian 'crusaders' start to pop up from Order cities (-1 population) and attack and raze all Evil and Neutral cities. Quantities of crusaders depend on the AC.
Fellowship of Leaves could 'return the world to nature'. At low AC, forests appear. Around 50, forests will regrow over where they were destroyed (destroying mines and towns), and animals start building more lairs. At high AC, jungles start popping up everywhere and we all know that's worse than hell terrain, even for elves
Council of Esus would be too fanatical about gaining wealth. Crime would increase according to the AC value. It would be interesting as you'll be fighting the crime counter rather than the AC directly. At high crime, riots start becoming common and improvements will be burned. At even higher crime levels, the cities will start revolting. Higher AC means that there's a random chance for an assassination attempt, ie. barbarian shadows spawn near a city and attacks.
Haven't really played the other religions yet, so can't say about them. Oh, and I'm not so versed in the vast FFH lore, so sorry if I misjudged something