When you return to the Dark Brotherhood Sanctuary, you will find Astrid waiting for you. She will tell you that it would be foolish to ignore what the Night Mother has instructed you to do. She tells you to head to the crypt Volunruud and meet with your contact, Amaund Motierre there.
Make your way up to Volunruud and enter the Nordic Ruin.
Just inside you will find Heddic's Volunruud Notes. This will start another quest: Silenced Tongues, which takes you through the rest of this Burial Chamber. After you read (or ignore) the note, turn to the left. There is a skeleton sitting in a throne. Attack and destroy this undead foe who is waiting for you to pass. After that, head on down the slope into the next chamber.
In this chamber you will want to go through the Western corridor. Follow it past the defeated Draugr Deathlord and other fallen Draugr. Go through to the next chamber. You will find Amaund Motierre here. He will give an outline of his plan. The Dark Brotherhood is to kill several targets ending with the Emperor. He will then ask his bodyguard: Rexus to give you a Sealed Letter and a Jeweled Amulet.
Leave the tomb and return to the Dark Brotherhood Sanctuary. Talk with Astrid to learn a bit more about what is going on. She will also direct you to head to The Ragged Flagon to talk with Delvin Mallory to fence the Jeweled Amulet. Talk with Delvin and he will take the amulet and give you a Letter of Credit, as per their standard agreement.