I'm looking for some advice for an upcoming saga. (Ars Baltica players, begone.)
I've decided that a rather nasty magus at the winter covenant the PC magi are joining has done something to a local river. That river, the Dvina, runs from the northern reaches of the Kievan Rus to the Baltic, and so was, historically, a major trade artery from Kievan Rus and Constantinople up to northern and western Europe. In the game, it's closed to shipping because of a nameless horror that lurks on it. The nameless horror, however, is the river itself.
The magus has awakened the river, and somehow driven it mad so that it kills most anybody that gets on or near it. The whole river valley is a nightmare land. The exact role of the infernal in all of this, beyond encouraging the nasty magus, is not yet decided. That is, I haven't decided whether the river's madness is demonic possession.
My question is about the river spirit itself. When looking at the forest spirit from GotF, and at the Rhine elemental plot seed from the same book, I'm left with no very concrete feel for how these sorts of spirits might behave. I'd like to know how the rest of you think area spirits think and act. What sorts of things do they want? Why do they want them? How do they get them? How do they think? Obviously, I know that I should do whatever's best for the saga blah-blah-blah. But the idea of an area spirit is sort of alien to me, so I don't really know what makes sense or will seem plausible. How have others handled this sort of thing?