Bixie Wrenchshanker, skulking just to the south of The Dens of Dying, has asked you to use Bixie's Inhibiting Powder at a Den of Dying Plague Cauldron to neutralize it.
1x Plague Cauldron Neutralized
So, while you were out taking care of those plagued beasts, I took a peek inside the caves up there.
There are cultists in there plaguing them!
They have some sort of noxious concoction brewing in a cauldron. I have a powder that I normally use as an anti-poison, and I mixed it up with some other stuff. I think it'll neutralize their brew!
Take it up to one of those caves and see if it works.