Astrid has given us permission to seek out Amound Motierre and talk to him as the Night Mother wanted us to. She even tells us that Volunruud is a dungeon to the northeast. Chances are that you haven't been there yet, so check out the map and you'll see its southeast of Morthal and north of Whiterun, up in the mountains.
Once you find the place, head inwards. At the beginning, you'll see a skeleton on a throne to the LEFT (be careful of him) and a journal on the ground called " Heddic's Volunruud Notes ". If you read it, it will start a new BIG quest called " Silenced Tongues ". You can do this quest if you wish (it's in our RANDOM quest section, since you stumble upon it); but to find Amound you don't actually have to go very far. Just down the steps and to the left.
Amound will be in a room with his servant. It seems that the reason you were to meet him was because he performed the Black Sacrament, and want to take out a number of contracts with you. Talk to him to realize the sheer SCOPE of who he want's killed...
You'll end up getting a letter and an amulet, which we need to take to Astrid right away, so leave this place and go back to the Sanctuary . Talk to Astrid and tell her what Amound proposes. The glory of it all will make her say ok, but first she will reflect on the letter while she sends you to go show the amulet to a fence named Delvin Mallory in the Riften Ratway.
Head over to Riften and to the ratway (which if you didn't know is in the sewer system under Riften...). Proceed to the Ragged Flagon (which you'll get to after a few fights if you haven't been here before) and inside follow the marker to Delvin. Go through the conversation options and accept the line of credit that he offers (worth 15,000 septims).
Now go report back to Astrid and give her the line of credit. She'll talk about the situation with you for a bit before giving you your next assignment: killing a bride at her own wedding! Oh, we're only working our way up to the big kill, don't you worry! This will end " The Silence has been Broke n" and start your next Dark Brotherhood quest " Bound until Death ".