Skyrim Dark Brotherhood Walkthrough: The Silence Has Been Broken

You get this quest automatically after doing Whispers in the Dark, and two contract kills for Nazir. Once you come back to the Sanctuary, Astrid wants to talk to you.

You can tell her:
Of course, Astrid, what is it?
Has the time come? Will I now serve the Night Mother?
(Remain Silent)

Try the second option. Astrid tells you something is happening here. She is not sure entirely what that something is, but we need to find out. If Night Mother really did give you an order to talk to a contact, we'd be mad to ignore it. And I think we'd both agree, Cicero's brought quite enough madness to this Sanctuary. So go. Go to Volunruud. It's a crypt, pretty far to the Northeast. Talk to this Amaund Motierre and let's see where all this leads.

Open the quest journal:

The Silence Has Been Broken
Astrid has agreed to let me see Amaund Motierre in the dungeon known as Volunruud, as commanded by the Night Mother. I'm to speak with Motierre, and find out what he wants.

Your objective is to talk to Amaund Motierre. Before leaving the Sanctuary, make sure to report to Nazir and claim your reward for the two contract kill missions. Fastest way to get there is to fast travel to the tower of Mzark, then go west. Reach Volunruud, climb down the stairs and go to a chamber in the Southwest.  Open the wooden door and find Amaund Motierre. He is thrilled to see you, the dreadful Black Sacrament thing had worked. You can tell him:
The Night Mother heard your pleas, Motierre
You have opened the door to darkness, little man
(Remain Silent)

Choose the first option. He tells you that he would like to arrange a contract, several actually. The work he is offering has more significance than anything your organization has experience in centuries. Tell him to go on. He wants you to kill several people. You will find the targets, as well as their manners of elimination, quite varied. He is sure that someone of your disposition will probably even find it enjoyable. But you should know that these killings are but a means to an end. For they pave the way to the most important target. The real reason he is speaking with a cutthroat in the bowels of this the detestable crypt. For he is seeking the assassination of.. the Emperor.

You can answer:
Leaders rise and fall. Business is business
You want us to kill... the Emperor. Of Tamriel?
(Remain Silent)

Choose the second option. He tells you that is correct. What he asks is no small thing, of course, but you represent the Dark Brotherhood and this is what you do. You must understand, so much has led to this day. So much planning and maneuvering. Now, it's as if the very stars have finally aligned.

He calls Rexus to give you items to be delivered to your superiors. You receive a Jeweled Amulet and a Sealed Letter. The sealed letter will explain everything that needs to be done. The amulet is quite valuable - you can use it to pay for any and all expenses.

Your quest journal is now updated:
Amaund Motierre has commissioned the Dark Brotherhood to kill several people, all of which lead to the assassination of a primary target - the Emperor of Tamriel himself. I am to report this information back to Astrid.

Your objective now is to deliver the letter and amulet to Astrid. Go back to the Sanctuary and talk to her. She asks you what Motierre wanted. Tell her Motierre wants you to kill the Emperor. She says you have got to be joking. Show Astrid the amulet and sealed letter. She now realizes you are not joking. To kill the Emperor of Tamriel - The Dark Brotherhood hasn't done such a thing since the assassination of Pelagius. As a matter of fact, no one has dared assassinate an Emperor of Tamriel since the murder of Uriel Septim, and that was two hundred years ago. Tell Astrid that surely the Night Mother wouldn't misdirect us. Astrid agrees that she certainly wouldn't. And, for whatever reason, she chose to relay Motierre's information to you.

Astrid does not know exactly what is going on here, if you're the listener, or this is some fluke, or what. But what we now have before us... Ask her if the Dark Brotherhood will accept the contract. She says yes. If we pull this off, the Dark Brotherhood will know a fear and respect we haven't seen in centuries. Astrid would not abandon an opportunity to lead her family to glory. But this is all so much to take in. She needs time to read the letter and figure out where we go from here. She mentions the Amulet. Ask her what she is thinking. She says she thinks we need that amulet appraised. She wants to know where it came from, how much it's worth, and if we can actually get away with selling it. And there's only one man who can give us what we need - Devin Mallory. He's a fence, a private operator. Works out of the Ratway in Riften. Astrid asks you for the letter, and tells you to bring Mallory the amulet and find out everything you can, and sell it if he's willing. He'll offer a letter of credit - that's fine. Devin Mallory and the Dark Brotherhood have history, so he can be trusted.

Your quest journal is updated:
I've given Astrid the amulet and sealed letter provided to me by Amaund Motierre. I'm to take the amulet to a fence named Delvin Mallory, in the Riften Ratway - to gain any information I can, and sell it if Mallory is willing to buy.

Your task now is to show the amulet to Delvin Mallory. Devin Mallory is part of the Thieves Guild in Riften, you might have already dealt with him if you joined the thieves guild. Go to Riften and enter the ratway, navigate through the sewers until you reach the Ragged Flagon. Find Devin Mallory inside. Tell him the Dark Brotherhood requires his services. He asks how Astrid is doing. Then he asks what is it the Dark Brotherhood needs from him. Ask him what he can tell you about the amulet and give Delving the amulet. He asks you where you got it, but changes his mind - he does not want to know. This is an amulet of the Emperor's Elder Council. Specially crafted for each member. Worth a small fortune. Ain't something you'd give up lightly. He says that it is not his business to tell the Dark Brotherhood its business, but if you killed a member of the Elder Council, you'd better belie...

Ask him if he will buy the Elder Council Amulet. He agrees. He gives you a letter of credit. Usable by Astrid only, for any service or item he can provide, as per their standard arrangement. He tells you to bring the letter of credit back to your lovely mistress, with his regards.

Read the update in your quest log:
Delvin Mallory has informed me that the amulet provided by Amaund Motierre indicates he is a member of the Emperor's Elder Council. Mallory was willing to purchase the amulet in exchange for a letter of credit, which I must now deliver to Astrid.

Head back to the Sanctuary. Speak with Astrid. Tell her the amulet is authentic and specially made for members of the Elder Council and give her the letter of credit. This concludes the quest The Silence has been Broken and immediately starts the next quest, Bound Until Death.