Stefan at Ebon Watch wants you to return to Drakuru and complete any tasks he has for you. While at the Reliquary of Pain, you are to use the Diluting Additive on 5 Scourge Cauldrons. If you lose your Ensorcelled Choker, return to Stefan at Ebon Watch to get another.
1x Drakuru's task complete
5x Blight Cauldrons diluted
Everything you've reported is consistent with what we know. Learning of Drakuru and earning his trust will lay the groundwork for your future missions.
Return to him and do as he says. Do whatever you must to appear sincere, [player]. A great deal is riding upon your performance.
This time I'm adding an additional assignment: While you're in the field, pour this additive into their blight cauldrons. It should neutralize the scourging agent and render the batch useless.