Garamantean Gothic

So, have you heard of the Garamantes? If not, google them right away. Or read The Fate Of Africa. I found out about them through the latter. I heard the part about extensive underground water-mining operations and my mind lit up with a big neon sign saying "DUNGEON". Plus, the whole concept of a cosmopolitan urban society being able to exist in the middle of a desert without extensive magic is pretty cool. 

Okay, plot idea. The Garamantean civilization is believed to have collapsed and dissipated when their underground water sources ran out, since they're non-renewable. So let's say that normally would have happened a century or two ago, but the fantasy-Garamantes king made a deal with dvergr/dero his spelunkers encountered that they would pump up water to refill the underground aquifers in exchange for the king secretly handing over indentured servants and prisoners of war to the derovergr as slaves. 

So, a few generations later, and the current king feels very guilty over this situation, or maybe the derovergr's demands are getting more and more unreasonable. So he wants to back out, but he still needs the water. So his plan is to bring in a famous magitek engineer to devise a new stable system of keeping the water flowing. But obviously the derovergr will be pissed if they find out about this, so the PCs have to first sneak the engineer into the city, then defend them and their workers while they build their system and collapse the derovergr access tunnels. 

You've got espionage, action, and lots of opportunities for both aboveground and underdark politicking- the derovergr aren't the only underground civilization, and it might only be possible to hold them back long-term by cutting a deal with another one. For example, by persuading the king to put pressure on his high priest to issue an edict declaring that ghouls are no longer unholy and can live openly and trade in the city, gaining the allegiance of an underground ghoul kingdom that can fight off the derovergr.