Online:The Wrath of Sithis

Quick Walkthrough
#Talk to Astara inside the Dark Brotherhood Sanctuary.
#Travel to the ruins of Knightsgrave and find Green-Venom-Tongue.
#Hunt down the Black Dragon and deal with her once and for all.
#Report back to Astara in the Sanctuary.

Detailed Walkthrough
Speak to Matron Astara in the Sanctuary and she will tell you that it is time to make a move against the Black Dragon. Green-Venom-Tongue has been given the assassination contract, and it is your job to provide backup and ensure the task is completed. Before you leave the Sanctuary, you can find Tanek up and about in the sleeping quarters, who will be able to give a bit of advice on your target location, Knightsgrave.

Leave the Sanctuary and head northwest. One option is to follow Varen's Wall all the way around until you reach the Colovian Revolt Forge Yard, and then just drop down to the coast. As Tanek suggested, the exterior of the ruin is guarded by Minotaurs.

Once inside the ruins, you need to locate Green-Venom-Tongue. Examine the statues of Cavor Merula, Justia Desticus, Rusio Olo and Amminus Entius on your way in for some background to the Order of the Hour's formation. At the bottom of the stairs, you will find the Argonian pondering how to open the door; you will need to light the braziers in the same order as the statues on the stairs: Cavor Merula, Justia Desticus, Rusio Olo, Amminus Entius.

When you enter the Path of the Order, a shadow of the Black Dragon will taunt you: ''"They sent you to hunt me down? I suppose that's what happens when all the good assassins have already been eliminated."

Green-Venom-Tongue explains that Lyra is using shadows to track your progress. Your objective is now to hunt her down, so proceed onwards past the minotaurs towards the Overlook of Time. As you approach the door, you may see the Wrath of Sithis float through the door, before the shadow of the Black Dragon appears again:
:The Black Dragon: "You'd kill me, Venom? You were my Brother once. Join me and forget your obligations."
:Green-Venom-Tongue: "Join you? I'm going to send you to the Void, traitor!"
:The Black Dragon: "The Brotherhood is a pox. It deserves to be destroyed."
:Green-Venom-Tongue: "What lies did Artorius fill your head with, Lyra?"

In the middle of the Overlook, Venom spots the Wrath of Sithis as well: "Assassin, did you see that? That wasn't one of Lyra's shadows. Like something out of the old stories we scared the initiates with …."

Lyra then appears to taunt you again:
:The Black Dragon: "The Brotherhood. The Order. I'm done being used, Venom. Just leave me alone!"
:Green-Venom-Tongue: "I can't do that, Lyra. You brought this on yourself."

When you enter the Chamber of Enduring Flame, Lyra pleads with Venom to turn back. Venom however climbs over a gate and continues without you.
:Green-Venom-Tongue: "Hmm. Interesting. It seems that gate blocks our way. One moment. I think I can get through that break in the bars."
:Green-Venom-Tongue: "Let's see if this switch opens the gate …. No, but it did open this other door. This is still my kill, Assassin. When one door opens, the best course is to leap right through."
:Green-Venom-Tongue: "Try to find another path. If you hurry, maybe I'll let you help me make the kill."

After Venom disappears through the door, the shadow of the Black Dragon appears again: "Follow me and you die, assassin. You can't defeat me."

As she fades away, two minotaurs break through the wall and you will need to defeat them before continuing. Circle around through the Order Armory to the other side of the gate that Venom disappeared through. Lyra and the Wrath of Sithis both appear again as you make your way through the room.
:The Black Dragon: "You are a determined one, I'll give you that. You must have really bought into the Brotherhood's lies."
:The Black Dragon: "I believed the lies once, when they told me we were family. Then they made me kill them."

When you leave the Armory, you find Venom lying on the ground, a sword through his chest.
:Green-Venom-Tongue: "I almost had her … but Lyra was always a step faster than me …."
:Green-Venom-Tongue: "I saw the Wrath of Sithis, Assassin. The Dread Father hunts her, too. Just like in … the old stories …."
:Green-Venom-Tongue: "You're the blade of Sithis … kill her and the Dread Father … will claim her soul …."''

Pick up Venom's Sanctuary Marginalia from the floor beside him and continue into the Chamber of Shadow. Lyra continues to taunt you, hunted by the Wrath of Sithis, as you pass through the Chamber of Shadow, and the Path of Light and Paragon's Crown beyond.
:The Black Dragon: "What did the Brotherhood give me? Nothing but guilt and shame."
:The Black Dragon: "The Order? They offered me redemption. But in the end it was just more lies."
:The Black Dragon: "And you know whose lies hurt the most? Primate Artorius."
:The Black Dragon: ''"Artorius isn't as pious as he pretends. You know, he even had me murder his beloved mentor."
:The Black Dragon: "And now I killed Green-Venom-Tongue, too."
:The Black Dragon: "I wonder, if we betray the Brotherhood, does the Wrath of Sithis really claim our souls?"''

After this last line, a door into a large circular chamber opens up, and you will now need to fight the Black Dragon's shadows.
:The Black Dragon: "Green-Venom-Tongue challenged me and failed. What hope do you have? Even my shadows are more substantial than you!"

Once the last shadow is defeated, another appears to taunt you again, and you must continue onward through the ruins.
:The Black Dragon: "Primate Artorius manipulated all of us. He led us to this cliff. Will you leap and continue to follow his lies?!"
:The Black Dragon: "I told Artorius the location of your Sanctuary. Brema was good enough to reveal it to me before I slit her throat."
:The Black Dragon: "The Order of the Hour will purify your Sanctuary. You cannot stop it."

At the bottom of the stairs, the ruins open out above a large cave with a dragon sculpture in the middle. The Black Dragon awaits beneath it.
:The Black Dragon: ''"And here we are. I'm ready to die. Let's see if you're good enough to kill me."

You will have to contend with the Black Dragon herself, as well as more shadows, during the fight. Once you have defeated her, the Wrath of Sithis claims her soul, dragging her down into the Void.
:The Black Dragon: "No! What's happening? The stories were true! Sithis, have mercy!"

You now need to escape Knightsgrave and head back to the Sanctuary to warn Astara that the Order knows its location. A ladder at the rear of the temple leads out to the Gold Coast.

After reporting in, the remaining members of the Brotherhood give their take on the events:
:Tanek: "So the New-blade brought down the Black Dragon? And I didn't expect you'd survive your first contract!"
:Hildegard: "All our Brothers and Sisters murdered. But we're safe now, right? The Black Dragon's dead."
:Astara Caerellius: "The Assassin ended the threat of the Black Dragon, but we won't be safe until Artorius joins her in the Void."
:Hildegard: "But the Primate speaks for Akatosh! Isn't that blasphemy?"
:Kor: "It can't be blasphemy if we don't adhere to that religion, Hilde."
:Tanek: "I want to plunge my dagger into him, once for every Brother and Sister he's killed."
:Astara Caerellius: "Artorius will die. You have my word on that. And you'll each have a hand in the slaying. Now move. We have preparations to make."''

Astara confirms that she needs to talk to the Speaker before you can move against the Primate. The final quest unlocks at Dark Brotherhood Rank 8.
 

Quest Stages