The Creche of Ammorichae
This is designed to be a conflict quest area, where completion of a particular set of events confers a bonus to the city that manages to succeed.
Goal: Defeat the monstrous entity Ammorichae and place her heart in an altar.
Reward: Successfully placing the heart into an altar gives every member of the city that does so an extra defense. I figured there could be three objects to place the heart in, and the benefit given depends on which they choose. Ideas for bonuses are +2% critical hit chance, +5-10% bashing damage bonus, +x% gold drops, or 10% xpgain (all of these would stack with whatever). The idea is to keep all the potential benefits as things which don't affect combat so there is no need to worry about balancing such things, and cities don't feel griefed if they don't often get it. They should be nice to have though, and beneficial enough that cities have a pretty decent desire to participate. Being the one that places the heart could also give an honours line, maybe something like: (He/She) has sealed the heart of Ammorichae.
Details: Cave system would have 10 creatures that roam around (the evil offspring of Ammorichae), are aggressive, and chase. These would be immune to critical hits and hit with room attacks where the damage is split evenly between everyone in the room, and maybe some annoying afflicts. This means they are easier to fight in a group, and if an opposing group shows up and a fight breaks out the spawns won't have a ridiculous affect on the outcome of the fight. Killing the spawns is entirely optional, but for each one that is killed Ammorichae takes an x% debuff to max health, making her easier to kill.
Ammorichae should be immune to critical hits, and also do room attacks with damage split between everyone present. Should be a challenging fight for 5 or so endgame characters, maybe a one or two less if they do a lot of debuff with the spawns. The reasoning behind that is that it will take a fair bit of time for a smaller group to clear the spawns, therefore the other cities will have more than ample opportunity to intervene if they want to. Ought to be basically impossible for any fewer than three super unicorns because there needs to be enough challenge to make it interesting as a conflict location.
Ideally the buff would stay on that city until the quest is completed again. This gives the city with the buff a good reason to show up if someone else moves in on it, and gives other cities a good reason to go and try to deny them the resource. Have Ammorichae only respawn every three hours. This means that a city winning the buff gets to make at least decent use of it before anyone can try to strip it, but it comes back up often enough that every city has a chance to try to win it several times a day, with particular cities peak hours for member login not mattering, because it'll come back around soon enough.
Only one city can walk away with the prize, which will hopefully reduce inter-city teamups, though this will surely still happen from time to time. Could possibly make it so the presence of one city within a tether blocks entrance from the other, eg. spines people inside, loch people can't enter the area. I don't particularly like that myself, but it could be an option if teaming is out of hand.
Ammorichae's room should be accessible by special entrance, maybe two on opposite ends of the caverns, so it is difficult for a team to win by just being annoying with walls etc. Her room's exits should all be rubbled and deposit people who make it out away from the entrances, so they have to walk back to rejoin the fight. The room where you put the heart in the altar is down from hers, and only accessible once she is dead. This makes it unlikely that someone could try to kill the person with the heart after they've done all the hard work. Probably have the heart go into the last hitter's hands automatically, so people can't be lame and try to snatch it when another org did all the heavy lifting.
I'm not particularly well versed on Aetolian lore, so I'm not quite sure what the origin of Ammorichae or the area could be. Maybe some sort of demon, or a creature created/summoned by the Ankyereans before the grand artifice for regular mortals to battle for their amusement? Feel free to toss out ideas.