Stefan at Ebon Watch wants you to use the Ensorceled Choker to apply the Scourge disguise. While in Scourge form, you are to purchase Bitter Plasma from Gristlegut. If you lose your Ensorcelled Choker, return to Stefan at Ebon Watch to get another.
There... I've created a disguise that should make you indistinguishable from the Scourge.
You'll be free to walk among them as long as the disguise holds. Only the blightguards will be able to see through my illusion. Watch out for them!
Another thing you must know is that the disguise is unstable and may drop unexpectedly, so stay alert.
Let's test it out. Use the choker and visit the Scourge merchant just east of here. Return to me with a bottle of bitter plasma.