Find an uncorrupted furbolg, and using the power of Dartol's Rod of Transformation, speak to him.
Alas, I'm not completely certain what he had in mind next.
If what he said was true about the rod's power, then it will allow you to take on another form altogether. Which form, I do not know.
Perhaps he meant to use its powers to get closer to the tainted furbolgs, but even the uncorrupted creatures would slaughter him were he not careful. Transformed by the rod, Dartol would have to speak to a furbolg who was safe from the effects of Fel Moss.
I would start your search southeast of Mystral Lake.