“ The Earthblood Warrens are a series of caverns in which dwell men who live around the Earthblood Vein, a river of magma. They use the magma from the Earthblood Vein to warm and light their homes and cook their food. The Warrens are several miles long, stretching along the banks of the Earthblood Vein, and since nobody wants to walk that far, the Earthbloodmen capture large, magma-swimming serpents, which they train and ride.”
“ 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...”