I simply love dungeons, handled correctly they are better than the Fruit Flavoured God of Awesomeness!
The heroes storm the Temple of R’gu, the God From The Stars, to steal the Ruby of the Winged Master
The Great King long ago ceased attempting to police the wastelands of his Border Marches, and these debatable lands fell into the hands of petty counts, retired generals, and warlords, who constructed and then abandoned many keeps ripe for plunder.
The Repository of the victorious dead…
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...