Chaos 3 (Sorcery)
Revelry Eternium
Very occasionally, a city earns the ire of an archmage of chaos. That city is often then only mentioned in hushed whispers on a night of storytelling, in an inn somewhere on pitch-black nights while winds howl outside (or in cautionary tales for the townsfolk or children). The stories vary from place to place, but generally follow this basic plot. The archmage, irked by rude service, the local scenery, or a very naughty child places a curse upon the city. The locals laugh it off over their drinks, until they begin to notice that laughing off things over drinks is all that some seem able to do. Soon, no one is noticing anything about anyone else at all, but all are having a great time. They drink, they make merry, they laugh. Oh, yes, they laugh. Men and women dance in the streets, murders occur over a bewitching look amidst the devilry, and shops are burnt down but everyone else is having too much of a good time to notice that something has gone very, very wrong. One by one, the citizens waste away (or are murdered) and all that is soon left are empty houses and city ruins.
Mechanics
Casting the spell affects the city in the following way. The city suffers a constant penalty to culture, gold and research, (suggested -10 culture, -50% culture, -10% gold and research), and every turn one 'unhappy face', representing a portion of the city incapacitated by the revelry, is added. The intention is to slowly incapacitate the city such that it starves to death or culture implodes, whichever comes first. Only one city of each civilisation (or one city in the gameworld, whichever makes more sense) may be affected by Revelry Eternium at any one time.
The curse may be removed by sacrificing a priest of any kind (confessor, ritualist, etc, with the priest removed from the game), or a spell that inquisitors or high priests could cast.
Wouldn't know the best way to programme or incorporate it, and I bet the AI'll haemorrhage trying to use it/deal with it, but I think it'll be a nice little nasty spell.
Mystic