Thieves Guild: The Lost Histories
After you have fenced 400 gold and completed the previous quest, you'll get a Journal update telling you to go see S'krivva about any special jobs. A thief named Theranis was sent to Skingrad to find a book called "The Lost Histories of Tamriel", but he hasn't been heard from since. You're to go find him and either help him recover the book or bring it back. This book is something that the Gray Fox wants very badly.
So get to Skingrad by whatever means you'd like and go hit up one of the beggars (20 gold this time unless they already like you). It seems that Theranis was in the Two Sisters Lodge and bragging about having stolen something from the castle. Unfortunately for him, the captain of the Watch was also present and hauled him off to jail. You aren't going to be able to bribe your way into the jail this time and there is no simple way to sneak past the jailor. You're going to have to find other options.
Option 1: get arrested. If you're looking for a quick-and-easy way into the jail, this may be it. Hey, it worked in "Two Sides of the Coin". Unfortunately, my game crashes every time that I try this approach (and I'm patched). I don't know whether this is a flaw in the game or a flaw in my system, but it might be worth a try on your part. Save first, though.
Option 2: ask around town. You'll find that you have a new conversation topic available, "Work at Castle Skingrad". Eventually you'll be pointed to Shum gro-Yarug, the Count's butler. You may have already run into him if you bought the house in Skingrad. Unless it was repaired by the patch (which seems to have broken a few other things), some players have encountered a situation where he is nowhere to be found. In a couple of cases, he fell off of a bridge or got attacked or something that caused his untimely demise. In other cases, he just isn't where he's supposed to be. If this turns out to be the case, you'll have to settle for option 1. If he's around, though, he'll hire you as a slop drudge. Your job is to feed the prisoners. Because of this, all you have to do is tell the jailor that you're the slop drudge and he'll unlock the door for you.
Option 3: take the direct approach. It's what we old PnP gamers used to call "out of character knowledge". Computer gamers call it a walkthrough.
Just in case you decide to do this legitimately, I'll walk you through the way the devs wanted you to do it. If you want to take the direct approach, just start at the bottom and work your way up until you get to Amusei.
All right, slop drudge. You're in the cell block, but there's only one prisoner and he's certainly not Amusei. His name is Larthjar and he doesn't want to talk to you unless you unlock his cell. So break out the lockpicks and open his cell door. He'll sneak out a couple of hours later and I've run into him on the roads a couple of times afterwards, barehanded and sneaking the whole way. Not sure where he was going.
Anyway, Larthjar will tell you that Theranis used to be here in a cell with an Argonian prisoner (do we know any Argonians with a tendency to end up in the slammer?), but that he was taken by the Pale Lady three days ago and hasn't come back. They never do after the third time and this was Theranis' third time. He'll also tell you that the Pale Lady came and took the Argonian prisoner a couple of hours ago and that he was beat up and bleeding pretty badly. You might have noticed the blood smears on the floors at the other end of the hallway? So head down that way and look closely and you'll see the outline of what appears to be a door. You'll also notice a "Strange Candlestick" that can be activated. Do it and the door will open into a secret passageway. The passageway will eventually end at the castle Wine Cellars, but the blood smears obviously go through it, so you have to, too.
Just keep following the blood and you'll wind up in front of a big wine cask (you know, an enterprising thief could make a small fortune off of all of the wine in this room). The blood smears appear to go into it, but there's no trigger immediately to hand. It's on the other side of the right-hand pillar (yet another "Strange Candlestick"). So into the wine barrel you go and into another secret passage. If you weren't sneaking before, now would be a very good time to start. Around the corner ahead you'll see the Pale Lady and what appears to be a dead body. If you've saved up any enchanted arrows, this would be a good time to use one. Otherwise, you'll either have to wade in for melee or try for a melee-weapon backstab. If you don't get her on the first shot, she'll turn invisible and come attack you. Yep, you guessed it. She's a vampire.
Once she's pushing up daisies (permanently - you know how these vampire-types don't like to stay that way), you can deal with the dead body (Theranis, but he doesn't have the book) and then deal with the Argonian. For those who completed "The Order of Virtuous Blood" quest, you're out of luck on collecting vampire dust from the Pale Lady. The Secret Cell key on the Pale Lady will open the portcullis. And inside is none other than Amusei. Seems he got nabbed while stealing a fish. Anyway, he shared a cell with Theranis, who told him about a treasure that needed to get to the Thieves Guild. But he's not going to tell you about it until you get him away from the castle. He promises to join the Thieves Guild after he gets out, though.
The best time to get out is during the early morning hours (after 2 a.m.). So wait, if necessary. You're going to retrace your steps until you get to the hallway with the blood smears. Rather than go left (the way you came in), you're going to go right until you get to the Castle Dining Room. There shouldn't be anyone there at this hour, but there are almost always one or more NPCs in there during the rest of the day. Just head across until you get to the door leading to the County Hall. Whatever you do, don't try to sneak down the big staircase and out the front door. You'll almost certainly be spotted. Instead, go left and out a side door. There may or may not be guards wandering on the courtyard balcony, but probably not. The devs seem to have removed a few guards in a few critical places to make it somewhat easier to get out safely. For example, the guard that normally stands at the entrance to the bridge isn't there. This is not to say that you can't get caught, because I have been - several times. Mostly because I was trying to get out at the wrong time of day.
Once you are about halfway down the road to the East Gate, you'll get a journal update telling you to talk to Amusei. He'll follow through on his promise and tell you to look for the book under a bush near the well behind Nerestarel's house (which is down the first walkway on the right when you come in through the East Gate). It's just laying there on the ground, but isn't there until you have that conversation with Amusei. So pick it up and hightail it back to S'krivva to collect your reward (400 golds). If fenced items are pressing and you took my advice about clearing out the wine cellar, head for your local fence and then head for S'krivva.
Your Gray Fox trivia for this mission is from the Lost Histories: