Eris is a naiad, beautiful and graceful, flirting with butterflies and whimsically governing her sparkling waterfall pools. But when the characters encounter her, she is a starved, ill-looking creature, emaciated, jaundiced and warty. Why is this?
Not far away, a detachment of ogre-warriors have set up camp. This could be part of a larger plotline (maybe they’re planning to attack a nearby town, or steal some cattle), but whyever they’re there, they still need to wee. And where better than some nice clear pools?
Defiled with the urine of the ogres, Eris’s pools have sapped her strength and she has degenerated into an ugly and sickly being, needing help.
She begs the PCs for assistance. Even the most kind-hearted of PCs would ordinarily recoil from this foul-smelling thing, but the promises she makes of recompense for helping her are too good to go unnoticed. She is very vague, as she does not fully understand what is wrong with her pools.
I leave it to the imaginations of the PCs to find a solution to the problem.