Guild Wars 2 – Disrupt the Nightmare Court at Brairthorn Den

“Speak to Warden Spy Alarin to gain access to Briarthorn Den. Once inside, defeat Nightmare duelists, reassure hounds, clear thorn barriers and help captives.”

Head to the North from Morgan's Spiral. You will find yourself in a very out of the way spot. This is where you hero will meet the Warden Spy.

Warden Spy Alarin: “Quiet. My disguise may be convincing, but the Nightmare Court is ever vigilant. These are dark times, they're taking villagers from Astorea and turning them to their cause.”
Hero: A) “What can I do to help?” B) “Why are you disguised?”
Alarin: A) “When you are ready, you can don the disguise and infiltrate Briarthorn.”
Alarin: B) “We know the Nightmare Court is taking people, but we don't know how they're turning them. But with so many undead around, we don't have the numbers for an assault. Hiding and observing is all I can do.”

Once you don the disguise your character will look notably different but nothing too bad. Feel free to talk with Veteran Ebon Knight Moith. It is a short conversation tree but interesting (and presents a Charm opportunity).

Moith: “Briarthorn Den is under my control. Tread lightly or I will have you vivisected and served to the hounds.”
Hero: A) “Who are you?” B) Charm - “How would have me serve the nightmare?”
Moith: A) “Such arrogance. I hold your life in my hands, and you insult me with your trivial questions? For such insolence I should festoon the gates with your entrails.”
Moith: B) “We need the sylvan hounds turned to serve our wishes. Punish them, and use the fear of pain to coerce them into joining our cause.”

To the West you will find Kennelmaster Marm there who you can talk to briefly.
Kennelmaster Marm: “I see you eyeing my hounds. Perhaps you feel drawn to training them?”
Hero: “I might. What's involved?”
Marm: “We beat them until they know only obedience and savagery. They must fear their master to the point of sacrificing themselves to avoid their master's ire.”

After that it is time to start exploring the area. Looking around you will find some Nightmare Sylvari that are called duelists. They wish to initiate the world to the agonies of reality. Talk with them and tell them you wish to begin testing this with each other. Talking with some of them, after the fight, you will be able to call them out for cowardice (adding some Ferocity).

Look out for how collections of thorn briars. Inside them you will likely find a small collection of Nightmare Pups. Interacting with the Pups you have the choice to either scold them (turning them more toward the Nightmare Court) or playing with them.

Continue to the North and you will find more Duelists, Pups, Hounds and Captives. With the Captives you have the chance to build your reputation. Just make a sweep through the area and you will easily complete the quest.

When you manage to complete the task you will get the letter “Withering the Nightmare” with 60 copper:
“I'm still impressed that you infiltrated the Nightmare Court. It takes a special kind of sneak to pill that off. My heart aches when I think about the terrible, terrorizing abuse they heap on the innocent. If they'd infected those poor souls with nightmare, it would have brought suffering to an entire region. You have our thanks.  - Alarin.”