From time to time, one of the All can be divided from the One and All mind. This is a traumatic event, and it is rare that the severed member of the All survives, let alone becomes a solitary One. But from time to time, it can occur. Seeking succor from the desperate silence in its mind, the One Beneath The Waves has sunken himself beneath the oceans of Shamash II, and there it lies, seeking fruitlessly to soothe its injuries.
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...