A while back, I picked up Lesserton & Mor from Faster Monkey Games. It’s a sourcebook for a vast ruined city (Mor), and the town inhabited by its descendants (Lesserton). Its approach is quite innovative, providing tables for the referee to develop Mor through random rolls. Although developed for a fantasy campaign, I believe it would fit quite well into my Mutants & Magic campaign (with some minor conversions).
In Mutants & Magic, Mor was an Ancient domed metropolis which continued to be inhabited after the Fall as an independent city-state until it was conquered by Beastmen. Lesserton was founded by the refugees, who then came under the domination of the Necromancer Kings until the founding of the Affiliation (see the Campaign Background for more info). I think I’ll place Lesserton and Mor near the Dismal Swamp (not too far from Krake’s Borough, actually).
In terms of game rules, the Tainted are actually near-humans and Lesserton Orkin are mutants instead. Halflings use the rules from my recent demihumans post. Instead of Orkin, it was a Beastmen army which conquered Mor (this is a source of much distrust against Beastmen in the Affiliation).
I recommend this supplement for referees looking for an interesting mini-setting. You can also download the free Player’s Guide to Lesserton. If I get a campaign going for Mutants & Magic, then Lesserton & Mor will certainly be an option for characters.