Skyrim:With Friends Like These...

Quick Walkthrough
#(Optional) Receive a mysterious note from a courier.
#Go to sleep.
#Kill a captive or the Dark Brotherhood's leader.
#Choose to join or destroy the Dark Brotherhood.

Detailed Walkthrough
A Mysterious Note and a Good Night's Sleep
thumb|right|The Mysterious Note
Some time after Aventus Aretino rewards you for killing Grelod the Kind and you've left Windhelm, a courier will approach you and deliver a Mysterious Note bearing the ominous words "We know." The next time you sleep for any duration, you will "awake after a full sleep" to find that you have been mysteriously transported to unfamiliar surroundings. The quest can also sometimes be started after completing Aventus's quest by sleeping in any bed, even before receiving the Mysterious Note from the courier.

Astrid's Test
You will awaken in an abandoned shack to a greeting from Astrid. She will calmly inform you that your killing of Grelod the Kind has pleased the Dark Brotherhood, but that, in doing so, you stole a contract belonging to the Night Mother, the Brotherhood's disembodied spiritual leader. Thus, you must repay your debt: "A life for a life", as Astrid explains. She will tell you that you cannot leave the room until someone dies. The unpickable door to exit the shack is locked, and Astrid has the key.

You have the option of fulfilling the Night Mother's demand for a life by killing Astrid. Doing this will cause you to fail this quest and immediately start the Destroy the Dark Brotherhood! quest.

If you don't wish to kill Astrid, the room is also occupied by three bound captives. Astrid will inform you that "One of these poor sods has a contract out on their life. Which one is it?" It is up to you to figure out which and then carry out the job. You can refuse to carry out the job, but you can't avoid killing somebody.

When facing them, the captive to the left-hand side is Fultheim the Fearless, a mercenary. He claims to be unaware of anyone in particular who would want him dead, but reasons that, being a sell-sword, any number of people could hold a grudge against him. When intimidated, he also admits, "Maybe there were... times I got carried away? But war is war."

The captive in the center is Alea Quintus, a crass and abrasive mother of six who constantly and irritatingly demands to be released. She indicates that the tough attitude she has adopted to manage so many children might motivate someone to wish her dead. ''"Do some people look down on me? Have I made some enemies? You're damn right."

The final captive, to the right-hand side, is a Khajiit named Vasha, who casually incriminates himself as an "obtainer of goods, taker of lives, and defiler of daughters."'' He is the only one of the three who remains calm in the face of imminent death, and mentions that this isn't the first time he's been "bagged and dragged." He also states that ''"If one of my enemies wouldn't pay to have me killed, I'd take it as a personal insult."''

In truth, it doesn't matter which captive(s) you choose. You won't encounter any of them again (unless you return to the shack, and even then you won't be able to learn more). If you kill one or more of them, Astrid will give you the password to the Dark Brotherhood Sanctuary's "The Black Door". You will then be given a key to let yourself out of the shack.



Enter the Dark Brotherhood Sanctuary
The Dark Brotherhood Sanctuary is located almost due south of the Abandoned Shack, near the southern border of the province of Skyrim and just west of Falkreath.

:Upon entering the Sanctuary, the optional Delayed Burial quest will fail if you started it, or be unavailable if you have not started it.'' If you want to do this minor quest, detour to the quest location, north of Whiterun, on the way to the sanctuary.

When you activate the Sanctuary's entrance, The Black Door, it will ask, "What is the music of life?" Answer with the password Astrid taught you: "Silence, my brother". You will be welcomed to your new "home" and the door will open.

Speak Again with Astrid
Find Astrid inside. She will welcome you to the family by giving you several pieces of Shrouded Armor apparel, which provide enhancements useful to assassins. (There are also several other Shrouded Armor pieces on a shelf to the right of the room.) You are a now a full member of the Dark Brotherhood, and enjoy all the advantages associated therewith.

Astrid will reveal that she is currently arranging an important job to give you, and that you should, in the meantime, take care of some routine assassination contracts handled by Nazir. She will encourage you to introduce yourself to your new family members and give a rundown on how the Brotherhood operates. She gives a brief history on the Brotherhood and also informs you that they have done away with all rules and policies except for one simple principle, "Respect the Family". You may also learn that the corpse of the Night Mother is en route to the Sanctuary, though she does not know when it might arrive.

She finishes by again directing you to Nazir, assigning you your next quest, Sanctuary.

Achievements
One achievement is unlocked at the end of this quest:
 

Notes
The key required to exit the Abandoned Shack will not appear in Astrid's inventory if you attempt to pick her pocket, but will only appear in her inventory if you kill her.
The amount of time you choose to sleep just before Astrid takes you is irrelevant—no time will pass and your "Hours Slept" statistic will not be updated. After waking up in the Abandoned Shack, the Rested effect will be applied nevertheless.
All the captives wear a unique headpiece that can be looted or pickpocketed; although it covers your eyes, you can still see while wearing it.
After Astrid's initial speech and before killing any of the captives, you can attempt to pickpocket her if you have the Misdirection perk. If you succeed, you can obtain a complete set of Shrouded Armor along with her Blade of Woe. At the end of the Brotherhood quest chain, you'll get another Blade of Woe as the reward, making it possible to dual-wield them. If you choose to spare the captives and kill Astrid instead after pickpocketing her equipment, she will be an easy kill as she obviously won't be wearing her armor or weapon.  On the other hand, if your pickpocket is high enough to steal these things, you're probably powerful enough in other ways to kill her even if she was wearing them.
The rewarded Shrouded Armor consists of a set of Armor, Boots, a Cowl and a pair of Gloves. This set is marked as Light Armor.
If you start this quest before receiving the Mysterious Note, it will never be sent to you.
An unused NPC with the placeholder name "Player Friend" is associated with this quest, but was not implemented in the final release of the game. Pre-release reports for Skyrim claimed that the Dark Brotherhood would make you assassinate NPCs whom your character had befriended, which this NPC's association with this quest corroborates.
All of the potential victims completely resist poison if the player reverse-pickpockets some onto them, so they must be killed using either a weapon or spell.
If the player chooses to kill Astrid, any or all of the three captives can be untied and released after Astrid's death. Fultheim the Fearless will be grateful: Alea Quintus will upbraid you as if you were her kidnapper, even though you're not: and Vasha will merely promise that he won't set his associates on murdering you.
Conversely, if the player kills a captive first, Astrid becomes "essential" and unkillable, so it is impossible to kill her to get the Abandoned Shack key off her. She can be knocked down, and will revert to non-hostile (and full health) when she gets up again), so the player can talk to Astrid to be given the key. 
Also, once one of the captives is killed, the player will never be given the option to release the remaining two - not even after leaving and re-entering the Abandoned Shack: they will remain there, bound and hooded, for all time unless you choose to murder them.
If you cast Clairvoyance inside the shack, it will point to a random captive, changing every time you cast the spell.
Ironically, given that they're already immobilized, all three captives are immune to paralysis effects.

Bugs
 
 
Sleep right after finishing the Innocence Lost quest (speaking with Aventus Aretino for the second time). This may trigger the courier's delivery or your awakening inside the shack.
 Open the console and type  . This should add the Mysterious Note to the courier, triggering its delivery and allowing you to continue the quest normally after sleeping.
 Open the console and type   to start the quest. Next, enter the command  . You will be transported inside the shack and the quest should proceed normally.
 
Reload a save from before she told you to kill one of the captives.
Open the console and type   and hit enter, then type   and hit enter.
 
 
 
 Open the console and type  . This should add the correct dialogue option. Astrid may not be inside though, you can find her in the Abandoned Shack, and she will have correct dialogue options.

Quest Stages