I find Acheron kind of boring as is, considering he is held. In every single one of my games, he is basically a late game happiness boost. You wait until you have a unit capable of killing him, then get some gold, culture and happiness resources in one of your cities. He doesn't even ignite the continent anymore, as he doesn't chuck fireballs at passing units.
It would be more interesting, IMO, if he acted a bit more like Orthus in that he was capable of ravaging the countryside. However, being as powerful as he is, he needs a different dynamic to Orthus.
Basically, I think you should toggle the held promotion on and off. When he is built, it is off as he needs to go and establish his hoard. He needs special AI that makes him fly to the nearest city, pillage the land, kill a unit or two and retreat back to his city. Maybe allow him to destroy a building or two inside a city. Then, every once and a while, the held promotion toggles back off and he goes on a mini rampage again.
It would be even cooler if he isn't built in a city either, but rather spawns at some point like Orthus does. Then he conquers one city and makes it his own. After that, he doesn't conquer any more cities, but just goes on random pillaging and unit killing escapades. Maybe make him a big enough nuisance that it makes sense for you to build a bunch of settlers and move away, like the Dwarves in the Hobbit, but that probably is not feasible considering how Civ works.
Maybe you could create an event when Acheron takes one of your cities that makes this a feasible option though (one of several obviously, so you aren't forced to take it). If you select it, it will disband all your remaining cities or turn them barbarian with 1 pop (so you can't immediately reconquer them as they will auto-raze) while keeping all their original buildings. It will give you settlers based on your existing population with the starting settler promo and some military units with the mobility promo so they can protect the settlers during the migration and move to a new area quite quickly. The idea is that Acheron would spawn quite early, basically about the same time the AI generally founds its second city, so there would still be plenty of land left for you to move to and the event wouldn't give you 10 settlers as your population wouldn't be very high yet.
It would be more interesting, IMO, if he acted a bit more like Orthus in that he was capable of ravaging the countryside. However, being as powerful as he is, he needs a different dynamic to Orthus.
Basically, I think you should toggle the held promotion on and off. When he is built, it is off as he needs to go and establish his hoard. He needs special AI that makes him fly to the nearest city, pillage the land, kill a unit or two and retreat back to his city. Maybe allow him to destroy a building or two inside a city. Then, every once and a while, the held promotion toggles back off and he goes on a mini rampage again.
It would be even cooler if he isn't built in a city either, but rather spawns at some point like Orthus does. Then he conquers one city and makes it his own. After that, he doesn't conquer any more cities, but just goes on random pillaging and unit killing escapades. Maybe make him a big enough nuisance that it makes sense for you to build a bunch of settlers and move away, like the Dwarves in the Hobbit, but that probably is not feasible considering how Civ works.
Maybe you could create an event when Acheron takes one of your cities that makes this a feasible option though (one of several obviously, so you aren't forced to take it). If you select it, it will disband all your remaining cities or turn them barbarian with 1 pop (so you can't immediately reconquer them as they will auto-raze) while keeping all their original buildings. It will give you settlers based on your existing population with the starting settler promo and some military units with the mobility promo so they can protect the settlers during the migration and move to a new area quite quickly. The idea is that Acheron would spawn quite early, basically about the same time the AI generally founds its second city, so there would still be plenty of land left for you to move to and the event wouldn't give you 10 settlers as your population wouldn't be very high yet.