Speak to Draeth V'Zaar within Scarstone in Cobalt Scar to begin this quest. You must be of evil alignment to be offered this quest.
Use the potion to test the actions of the cursed bones on New Combine Tombs around 4354, -821, 330 . 2 zombies will spawn. They have a whole lot of hit points but don't hit very hard
Return to Draeth who learned that the bones came from a dragon killed by a very powerful necrotic spell which killed the dragon AND contaminated the bones (same ones as in the Othmir village) which are, in turn, spreading the disease.