Twilight was still getting over the disgust of the contents of "A Kiss, Sweet Mother" when she went to bed.  During the night, Gabriella took Twilight Sparkle from her home and took her to the new Sanctuary that had been created by Festus Krex.  Twilight didn't wake up until morning, and when she did, she was curious as to why she felt like she was on stone.  When her sight began to clear up, she realized that she wasn't in her library anymore; she was somewhere underground judging by the appearance of the area around her.  It was then that she saw Astrid looking at her, but in armor that she did not recognize.  However, since there was a mask covering Astrid's face, Twilight didn't recognize her.  The armor looked like this:

Twilight was so filled with fear that she was practically soaked in her own sweat.  Astrid merely said, "Sleep well?"  

"What?  Where am I?  Who are you?"

At this Astrid merely replied with, "Does it matter?  You're warm, dry, and still very much alive."  Twilight was relieved to know that she was still alive, but she was still curious to know where she was, and to her question, Astrid replied with, "You are in the Sanctuary of the Dark Brotherhood.  We have to thank that copy of 'A Kiss, Sweet Mother' for us to find you, we placed a detection spell on it."

Twilight was now completely terrified to the point that she was afraid her heart would stop.  She was in a Sanctuary for assassins.  "Why am I here?  What did I do?"

Astrid replied with, "You're here because we need new members to our Family.  We only just arrived in this world from our own, to bring the glory of Sithis and the Night Mother to your world.  You can help us.  Your intellect and skill with magic, as well as your connections to your ruler could help the Dark Brotherhood bring its glory to your land of Equestria in its fullest."

"WHAT?!  Oh pl-please...  I don't want to kill anypony.  I don't want to take innocent lives!"

Astrid's look of amusement was only hidden by her mask, and she said to Twilight, "In our Family we have a saying, 'Innocence is life's greatest illusion.'  You wouldn't be killing innocents, you would be killing people who ruined other people's lives.  And by 'people' I mean ponies, griffins, changelings and dragons alike.  Your contracts for assassinations would be helping other people's lives, and you would be payed with riches beyond your wildest dreams."

Twilight was somewhat in-between a state of joy and a state of disgust.  The Dark Brotherhood would justify their killings of other people as 'helping' or 'simple business' and all the money earned would be blood money.  However, she couldn't deny that the money would help her and her friend's lives.  With the money, she'd be able to purchase new books and renovations for the Ponyville Library, and she and Spike would have less to worry about financially.  However, Twilight had one question, "If you used that copy of 'A Kiss, Sweet Mother' to track me, then why do I still have it?"

Astrid replied with, "You still have that copy because you are going to publish it in Ponyville, and you will also publish those other copies you made of it to other libraries in Equestria.  You will be helping us make our presence known, and in addition, we will be spreading other works of literature to explain our history and our Family to others.  Do this for us, and we will give you the funds to improve your home and help your friends."

Twilight thought deeply on this offer.  Eventually she decided that she would become a member of the Dark Brotherhood.

Astrid was pleased with Twilight's decision, and she said, "Welcome to the Family, dear Sister Twilight.  But first, a welcome home present, the armor of the Dark Brotherhood.  May it keep you safe in all your endeavors."  

Then, Twilight Sparkle was given a set of Dark Brotherhood armor identical to the set that Astrid was wearing.  As soon as it was in her hooves, Twilight put the armor on.  "This armor looks pretty good on me.  I think I could get used to it," she thought to herself.

Astrid then gave her a tour of the Sanctuary, but not before she removed her hood, to which Twilight was utterly shocked.  "ASTRID?!  YOU'RE THE LEADER OF THE DARK BROTHERHOOD?"

Astrid smirked at Twilight's shock, and she replied with, "I am leader of this Sanctuary in your world, and a Sanctuary back in my world.  Arnbjorn and the others are also part of the Dark Brotherhood.  Now, I shall tell you why me and the others were so shocked to hear Shadowmere talk.  You see, back in our world, the planet is ruled by creatures called 'humans' and also inhabited by elves.  Horses in our world are used for transportation and they are unable to talk.  Shadowmere was a horse that was created by a being called Sithis, and Shadowmere has been a noble steed for many of the leaders in the Dark Brotherhood's history.  He looked similar to how he looks in this world, only... different.  When we came to this world, we were utterly shocked to hear him speak because he was unable to speak in our world."  Twilight definitely thought this story to be interesting.

"Before you go, our Sanctuary is just beneath the road near the Everfree Forest.  When you come back here, when questioned by the Black Door, speak the phrase 'Innocence my Brother' and the door will allow you entrance to our Sanctuary."

Twilight replied with, "Thank you Astrid, I will certainly come back once our presence is known and the contracts start coming.  I'd better get to work on publishing 'A Kiss, Sweet Mother.'"

After Twilight got back to her library, she decided to write a letter to both Princess Celestia and Princess Luna.

Dear Princess Celestia and Princess Luna,

Yesterday, 9 new ponies arrived in Ponyville, and they all had the exact same cutie mark, a black symbol that they called a "hand".  I have a sketch of the mark on the back of this letter.  Even more shocking is that one of the ponies was reptilian, and one was a filly who is actually a living breathing vampire!  They seem to be from a place they call Tamriel, and instead of using your name or Luna's name in their speech, they use the name "Sithis."  Perhaps you and Luna should come to Ponyville and meet them in person.

Your faithful student,

Twilight Sparkle.

P.S.  They are able to eat meat of all things!  Can you believe it?