“ Shadows, Mirrors and Flames. Three things we have all seen and not given much thought to. What if in each of these things you could find entire alternate planes of existance? Shadows move but what they are shadowing does not. Mirrors show us what others can easily see. Flames dance and twirl inviting you to dance only to scorch you when you get close. These three things hold much mystery. What if you could harness their powers? Heh...”
“ 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...”
“ The spellbook of a long dead wizard that has gained the ability to read itself and cast spells from itself. If could be amazingly powerful as an ally, or use its magic to manipulate and rule a kingdom.”