This quest is not available in game.
Bring the Brain of the Black Stalker to Khn'nix at Sporeggar in Zangarmarsh.
Historically the greatest danger to my people has been from the marsh walkers. With the new problems we are facing from spore giants, we cannot afford another attack from our ancient enemies.
You have proven yourself to be capable, <class>. Do you think that you'd be able to journey to the Underbog in Coilfang Reservoir and eliminate the great mother of the marsh walkers, known as The Black Stalker?!
Bring me her brain as proof.