Also known as the Splendid Forest Serpent, the Dragon of the Barrens, and the Keeper of the Mysteries of the Ssesha
Near the end of many a heroic venture or quest a brave band of adventurers upon return will find it necessary to seek out a sage, a wisened person, looking for the answer to a riddle, to decipher a map, or to unlock a geas or curse placed on one of their own. The strangest of quests, those touched by the madness of the fae, the eldritch influence of the outsiders and abominations, and those tainted by the hellish realms, are best met by the mysterious sage known only as Mugwort.
In the dank,dense areas of the world where the vigorous and the decaying are intertwined and indivisable, lives the wise bagabond. But getting him to impart his knowledge is quite a chore.
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...