“ 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...”
“ Dwarven Mountain Pony - a small, and hardy animal well suited to living in the mountains and highlands. Able to subsist on grass and brush, with hard hooves, and nimble feet. The dwarves use them as pack animals, and to pull ore carts, and to drive bellows for their furnaces. Never ridden as dwarves like to keep both feet firmly on the ground. As stubborn and tough as the dwarves themselves. Also only about 12 hands tall (1 hand=4 inches)”