With Friends Like These...

It is immediately preceded by Innocence Lost, and is a continuation of those events.

Quote
"Make your choice. Make your kill. I just want to observe... and admire."

- Astrid

Objectives

 * Kill one of the captives
 * Speak with Astrid
 * Enter the Dark Brotherhood Sanctuary
 * Speak with Astrid

Walkthrough
After the Innocence Lost quest has been completed, a courier will appear and give the Dragonborn a note with the imprint of a black hand and the words "We Know" written beneath it. The courier generally takes between one and three days to arrive, depending on how much the Dragonborn moves around the lands of Skyrim.

Once the note is received, the quest begins by sleeping in any bed. Note that the bed cannot be in residential Windhelm (where "Innocence Lost" is obtained from Aretino), though sleeping in the Palace of the Kings should begin the quest.

The Dragonborn will awake in an abandoned shack, located in the marshes north of Morthal. A Dark Brotherhood assassin named Astrid will speak to them. She reveals that the Dark Brotherhood is aware of their murderous deed and is pleased that Grelod is dead, but since the Aretino child performed the Black Sacrament, they demand that a debt be paid for the kill that was stolen from them. Astrid has brought three other people to the shack where the Dragonborn awoke. Fultheim the Fearless, Alea Quintus, and Vasha are tied up and wearing execution hoods, but are able to answer questions. Astrid tells the Dragonborn that there is a contract out on one of their lives, and the Dragonborn must figure out who to kill.

Killing any, two or all of the three will cause the Dragonborn to be rewarded with an initiation to the Brotherhood regardless of his or her choice.

The quest continues in the Dark Brotherhood Sanctuary, where speaking with Astrid completes the quest. She gives the Dragonborn the Shrouded Armor as a reward, and the next quest Sanctuary begins.

Alternatively, choosing to kill Astrid will fail With Friends Like These... and begin a new quest, Destroy the Dark Brotherhood! This will prevent any further progress in the Dark Brotherhood quest line. The Abandoned Shack cannot be left without killing at least one of the three prisoners or Astrid.

Quotes
Each kill or combination of kills evokes a different response from Astrid upon speaking to her afterwards.

Killing Vasha
"The conniving Khajiit. Cat like that was sure to have enemies. It's no wonder you chose him."

- Astrid

Killing Alea
"Ah, the feisty goodwife. Quite the mouth on her. Someone must have wanted her dead... right?"

- Astrid

Killing Fultheim
"Oh ho. The whimpering Nord, eh? Yes, yes, I can see how you'd come to that conclusion. Interesting choice."

- Astrid

Killing Vasha and Alea
"Hmm. The fearless wife and mother, and the underhanded Khajiit. Had to be one of them, right? Interesting choice."

- Astrid

Killing Vasha and Fultheim
"The timid warrior and the sly Khajiit. Must have been one of them, right?""

- Astrid

Killing Alea and Fultheim
"The blubbering barbarian and the tenacious mother. Must have been one of them, right?"

- Astrid

Killing all three
"Well, well. Aren't we the overachiever. Three possibilities, three victims. Must have been one of them, right? So why take chances..."

- Astrid

Killing Astrid
"...Well....done."

- Astrid

Rewards

 * The Shrouded Armor from Astrid, either as a reward for killing the target she requested, or looted from her body if she is killed.

Trivia

 * The bound captives make it easy to level up non-destructive skills (e.g. Illusion, via use of calm/fear/fury spells) indefinitely. Astrid also makes it possible to level up easily, as she will not continue to attack when defeated, and will just stand there until she is attacked again.
 * Clairvoyance will always reveal a target selected by random, the first time the magic is cast.
 * Though the three captives can not see due to the hoods over their faces, they still seem to be able to tell where the Dragonborn is, seeing as their heads follow them as they move about the room.
 * The black hand on the courier's note may be a reference to mafia syndicate La Mano Negra("The Black Hand"), which used the same symbol when sending threatening letters to victims. In Elder Scrolls' lore, it's the symbol of the Black Hand of Mephala (symbolic itself of the Dark Brotherhood's Listener and Speakers ((thumb and fingers)), Daedra Prince of Murder, and patron Daedra of the Morag Tong, the predecessor of the Dark Brotherhood.
 * The execution hoods, like other clothing, can be pickpocketed. They will reappear after reentering the cabin if any captive is still alive.
 * Viewing the world map will place the Dragonborn's marker wherever they slept before waking up in the shack. Upon leaving the shack, the map location displays correctly.
 * Attempting to fast travel from inside the cabin will bring up a message that there are enemies nearby.
 * Up to three execution hoods can be looted off of the bodies and equipped, however, the Dragonborn can still see perfectly while wearing an Execution Hood.
 * The abandoned shack is the only place where the Dragonborn can find the black execution hoods, barring the use of console commands.
 * Any follower will not be present inside the shack, but will reappear immediately upon leaving the shack.
 * If the Dragonborn uses the Wabbajack to turn a hostage into an animal or into any other form, it will count as killing them, even before the effect wears off. When the effect does wear off, the victim will be dead.
 * If the Dragonborn kills one of the captives, talks to Astrid, then leaves and re-enters the shack, Astrid will be gone. Even though there is only one door the Dragonborn will never see Astrid leave.

Bugs

 * When looking at the map, the Dragonborn may be shown in where they slept, not in the Abandoned Shack.
 * If the Dragonborn kills one of the victims and raises them as a zombie, Astrid will  become unavailable for conversation, barring progress in the quest. The only way to exit the shack then is to kill Astrid. (Killing another victim and not raising them does not fix this).
 * Sometimes it can take several days to receive the courier's letter.
 * If the courier still does not appear, try fast traveling to a different hold and waiting.
 * Taking the hoods off the victims will affect their ability to see the Dragonborn, but will not influence their dialogue or attitude (i.e. they will still speak in the same muffled manner and refer to being unable to see).
 * After killing Astrid, if the Dragonborn speaks with any of the prisoners but exits the dialogue without freeing them, they will respond with unmuffled voices, even though they are still hooded.
 * Killing a prisoner and leaving the shack before completing the quest will cause Astrid to teleport back to the Dark Brotherhood sanctuary.
 * Sometimes upon waking up in the shack, the blur effect will not trigger and Astrid will start talking normally.
 * Sometimes after waking up, the Dragonborn will be unable to move after the conversation with Astrid. Reloading from a previous save does not fix this. Using the Wait command may fix the problem.
 * Using the console to unlock the Abandoned Shack early and killing Astrid will prevent any quests related to the Dark Brotherhood from being completed.
 * After receiving the note from the courier, sleeping may cause the game to freeze.

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