There is an Underground World with its own Sun
Under the surface of the Ninth World is a massive globe-spanning tube, so vast it has it's own sun and moon. Biomechanical airships ply the airways and races unknown have lived and died there for untold ages.
By putting the setting under the player character's feet, it allows the GM to jump to this world from wherever their campaign is set. By making it a tube, it can exist side by side with other underground settings, in case Monte gets a shovel.
Urbamorr lurks on the Side of this World
This is the city, and it really is on the edge, and in more than one way. A city mostly in ruins, with a ruling class that spends its time in dreams, while their city crumbles and mind-controlled slaves toil to keep it going for one more day. Year. Eon.
Motivations to Explore this world: Discovery and Rescue
A whole new world provides endless opportunities for exploration and discovery. And to make it personal, the player characters are also presented with a kidnapping they can solve. Twin grandchildren of a prominent Aeon Priest have been spirited away to Urbamorr. What's the best way to rescue them, diplomacy or confrontation?
Choices Abound
Once in Urbamorr, the player characters have to make choices. Who can they trust? Is the enemy of my enemy really my friend? Is the enslavement of mind-controlled races really so different from the shackles of slavery on the surface?
Some groups will approach this as a leadership and diplomacy challenge. Others will leap into action. Still others will mix the two, and the adventure gives the GM the tools to make the adventure fun and exciting regardless.