With Friends Like These...

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

Background
See: Innocence Lost

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.

Once the note is received, sleep in any bed.

Note: If you sleep after completing Innocence Lost, you will begin this quest whether you have received the courtier's message or not.

The Dragonborn will awake in an Abandoned Shack, located in the marshes north of Morthal. A Dark Brotherhood assassin named Astrid will speak to the Dragonborn. She reveals that the Dark Brotherhood is aware of the 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 an execution hood, but 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.

To join the Dark Brotherhood, kill any combination of the three. The Dragonborn will be rewarded with an initiation to the Brotherhood regardless of their choice. Alternatively if killing Astrid will close the option to join the Dark Brotherhood and a new quest, Destroy the Dark Brotherhood! will begin. The Abandoned Shack cannot be left without killing one of the three prisoners or Astrid, so if the Dragonborn cannot kill Astrid, they must kill any combination of the three prisoners.

Kill Choices
Each kill, including killing all three of the possible victims, invokes a different response from Astrid upon speaking to her after you have done so.

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

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

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

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...'"

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

Killing the Khajiit and the Nord warrior:

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

Killing the Nord warrior and the mother:

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

Killing Astrid:
"'...Well...done.'"