Ancient ruined city?

[Ultimo]

So, I'm working on an AE arc, and I wanted to have the final mission happen in an ancient ruined city something like HP Lovecraft's Elder City in Antarctica (think, dark, dusty corridors of smooth black stone).  I've been trying to find a map that has that kind of look, and haven't come across one.  Does anyone have any suggestions?

 

Also, I wondered if there was a way to make an objective spawn only after a series of boss objectives has been completed (ie. there are 7 bosses you have to defeat in order for the final boss to appear)?  At present, I seem able to link the final boss to only ONE other boss objective, which would mean he spawns before the OTHER bosses are defeated, if you happen to fight the linked boss first, which is not appropriate for the story.  I need him to spawn after ALL the others are defeated.