"They said it would bring us a new age of wonder, of exploration, of excitement. I don't think this is what they meant: us scurrying around like rats in our cities of steam and steel, far away from the land and the sun."
An industrious colony of Gnomes have managed to turn a hostile environment into a bread basket.
The People of the pocket realm of Brocschtal are simple folk who live as they have for thousands of years. Farming the land, raising sheep, getting in the occasional brawl. And fighting off the infernal attacks of ghouls.
They should have spoken up sooner and saved poor Harold from certain embarrassment.
The world was ripped apart in a great cataclysm 3,000 years ago. This is a Codex of the pocket realms created by that great sundering.
Three cities that come and go, yet never see the shining sun.
The Imperial city has been sucked into hell and the rest of the world has been ripped apart. And tying it all together is the Crystal Tower. The Tower lives in all realms, a needle piercing the fabric of each reality and threading them all together.
With a huge number of sacrificial victims, another realm long ago secretly bound a Prince of the land spirits, Iorstonn by name, thus ensuring the fertility, and improving the magical defences, of their realm. But binding that Prince to one spot has disrupted the weather and magical patterns (ley lines?) over half the continent. Things have become bad enough that a coalition has been formed to rectify the situation, once divinations have established the cause.
Your party is one of the teams assembled to do the rectification: find the binding object and steal it ("so that we can destroy it here in our realm, of course"...*) or destroy it over there. Presumably the Prince will be grateful to his rescuers and angry at the binders, but with a greater spirit like Iorstonn one can never be sure: such beings are "tricksy".
* Of course, the rulers of our realm might want to control Iorstonn for their own ends...