Activating the quest
Chance to activate this quest will show up after completing the quest Whispers in the Dark, but additionally you're required to complete at least one of the side contracts ordered by Nazir. Return to the Dark Brotherhood's Sanctuary and right after you get there, Astrid should run up to you. The assassin will now allow you to go to the Volunruud crypt in order to find Amaund Motierre, who was mentioned in the previous quest by the Night Mother.
The Dark Brotherhood unlocked quest: The Silence Has Been Broken
New primary objective: Speak with Amaund Motierre
Leave the Dark Brotherhood's Sanctuary and open the world map. The Volunruud crypt you're looking for is located in the central part of Skyrim (screen above). Assuming that you've not encountered this place yet, it is worth to begin your travel in one of two places. You might go to Whiterun and then head north or go to the Loreius's farm (location where first quest associated with the Dark Brotherhood takes place) and then head west.
The Volunruud crypt fortunately is not hidden high in the mountains, and its entrance is located inside a large hole in the ground (screen above). Go down and use the small door to enter this underground construction. Move to the north-west. You might attack the skeleton you're passing by before it gets back to life, but fortunately it is the only opponent on your way to the destination. When you reach a junction, turn left choosing a passage leading to the southwest.
Enter the small room with dead draugr and open a door to the right. You can now approach Amaund Motierre in order to talk to him (screen above). You'll learn from him that he wants to give few contracts to the Dark Brotherhood and last of them is a task to assassinate the Emperor of Tamriel himself. You'll get from your interlocutor an Amaund Motierre's sealed letter and a jeweled amulet and you'll be asked to deliver these items to your supervisor. If you're curious type, you can open the sealed letter right now and learn a bit more about the assassinations you'll commit in the future. Breaking the seal won't affect your relations with the Dark Brotherhood.
New primary objective: Deliver the letter and amulet to Astrid
You can now exit the Volunruud crypt the same way you came here and then go to the Dark Brotherhood's Sanctuary. Once you get there, look for Astrid and start a conversation with her. Inform her about your talk with Amaund and give her items you've received from him (screen above). The assassin will ask you to visit her friend named Delvin Mallory in order to price the amulet.
New primary objective: Show the amulet to Delvin Mallory
Leave the Dark Brotherhood's Sanctuary, open the world map and go to Riften located in southwestern part of Skyrim (screen above). Delvin Mallory can be found in the Ragged Flagon inn and if you already know this place (you might have visited it during completing one of the primary quests and shortly after starting cooperation with the Thieves Guild), you can skip this part and jump to the section discussing the conversation with Delvin. Otherwise, see below.
In order to get to the Ragged Flagon, you have to pass through the Ratway. The entrance is located on the lower level of Riften. I suggest using the stairs south of the passage shown by the game (screen above). After getting downstairs, just walk towards the door and you'll be moved to the Ratway.
Start exploration of the Ratway. After few moments you'll encounter Hewnon Black-Skeever and Drahff (screen above). The game can let you go without the fight, by using highly developed Speech and relying on the persuasion, but more likely scenario is that you'll be attacked by these bandits. Adjust your tactics depending on your current opponent. Due to the fact that you're fighting in the narrow corridor, you might use the Shout Unrelenting Force. Examine corpses of defeated enemies and continue exploring the sewers. Ultimately you have to reach the raised drawbridge. Ignore it and jump down to the lower level.
There are two paths leading to the next location. Using the shorter one involves opening a well-protected eastern gate (screen above). If you're not able to open it or just want to thoroughly explore the area, then use the western door. Watch out for the first trap behind this door - spikes which come out from the side when you open the door.
If you've chosen a roundabout path, then you have to reckon with eliminating skeevers and Gian the Fist (screen above). The latter one prefers melee combat, so if he makes any trouble, try to attack him from the distance. In the room where Gian was staying, you have to watch out for bear traps, bypassing or disarming them. Fortunately even if you step on one of the traps, you won't lose too much health. There is one more trap on your way, connected to pressure plate on the stairs leading to the upper level.
Both paths described above lead to the same room, where you'll have to get rid of vagrant. After the fight use the door leading to the Ragged Flagon (screen above).
After getting inside the Ragged Flagon inn, locate Delvin Mallory and talk to him (screen above). During the conversation you'll learn that the jewel you got from Amaund is very valuable, because it belongs to one of the Emperor's Elder Council. Despite an enormous risk of possessing such an amulet, Delvin wants to purchase it from you, giving you a letter of credit in exchange.
New primary objective: Report back to Astrid
You must return now to the surface and therefore walk again through the Ratway. The only difference this time is that after getting to the room where you've killed vagrant, you'll have to choose the northern passage and interact with lever lowering the drawbridge (screen above).
Open the world map and confirm that you want to travel to the Dark Brotherhood's Sanctuary. Once you get there, look for Astrid in order to give her a letter of credit and getting the first contract from Amaund's letter (screen above). If you've already opened the letter, Astrid will mention this fact, but you won't be punished because of this. The assassin will describe you details of first contract. You have to go to the wedding organized in the Temple of the Divines in Solitude and assassinate the bride - Vittoria Vici. It is worth mentioning that you can get a bonus if you kill her during one of her speeches. You can now visit other assassins present in the sanctuary, asking them about ideas of assassinating the bride. The most useful clue is offered by Gabriella, who tells you about "presents" left on one of the balconies near the wedding. I suggest visiting Nazir too, because the game will unlock the possibility of completing all other optional contracts.
The Dark Brotherhood unlocked quest: Bound Until Death