To unlock this achievement you will need to complete two quest this time after your last promotion. The first being 'Lost Histories', the second being 'Taking Care of Lex'.Lost Histories
Before starting this quest you will need to fence at least 400 Gold in items to Luciana Galena, this will be the first time in fencing to her. I first found her in Bravil just slightly to the West of Castle Bravil, but to avoid confusion here is a bit of info about Luciana from http://www.uesp.net/wiki/Oblivion:Luciana_Galena
that helped me: "At midday she leaves and enters the lodge for a two-hour lunch. At 2pm she positions herself behind the bar where she will stay for a whopping twelve hours, helping out innkeeper Bogrum Gro-Galash and socializing with the regulars. She heads back to her house and finds her bed at 2am. Both training and bartering services are available 24 hours a day, but Luciana will take her precautions and deny you any illegal activities while she is outside."
Im only posting that info here because when i first met Luciana she was outside and i couldn't figure out why she would not let me sell items to her. This may not be a problem to most players but just incase it is, at least this will help anyone else out quickly.
Once you have fenced enough goods you need to speak to S'Krivva (at the time i found her in the same lodge as Luciana) and talk about the missing theif called Theranis, then i would highly recommend using this guide for the quest if you are stuck:http://www.uesp.net/wiki/Oblivion:Lost_Histories
I used the method of taking the 'Slop-Drudge' job as it seemed simple, which it was. As for escaping that didnt go to well, i was spotted straight away and had to run through the castle and over the narrow bridge until the Argonian caught up with me. After speaking to him i then had to travel by horse to find the book and then all the way back to S'Krivva to finish off the quest, but was then able to clear my bounty with S'Krivva. So if you do a good job of sneaking out of the castle you can still fast travel to the finish.
NOTE: Even though i cleared my bounty i still could not fast travel or 'wait', im not sure what i had done wrong. So to resolve this i first fenced my items to Luciana after arriving back in Bravil and then pickpocketed a guard near the gate. Then i payed the fine and was able to fast travel again, but if anyone knows why i could not still Fast travel after clearing my bounties i would love to know!
Also when you pass through the Wine Cellar that you cant miss, steal plenty of Wine! This will help you towards your next lot of fencing, the bottles are not heavy and you can grab a decent amount of bottles that are worth 2-8 gold each.Taking Care of Lex
Before starting the next quest 'Taking Care of Lex' you need to fence 500 Gold worth of items to the same person (Luciana). Then speak to S'Krivva to begin the quest. Use this guide if necessary to help you if you are stuck:http://www.uesp.net/wiki/Oblivion:Taking_Care_of_Lex
Following the guide i ran into trouble straight away again with the guards, as soon as i went through the secret passage that Orrin opens up for you. I reached the end and as expected there was a guard to the right patroling the corridor, no matter what i tryed i could not sneak past this guard at all! Maybe it would have helped with a invisibility spell or a higher sneak level but no matter what i did he would spot me and warn me everytime. According to another oblivion website the guard is facing away from the passage exit, regardless of which way he was facing personally he saw me everytime! So eventually i gave up and carried on to the door i needed to unlock, after picking the lock the guard tried to arrest me.
Obviously i resisted arrest, stole the letter and ran back down the passage timing it well to close the passage door behind me. I then ran to the fighters guild (which i was already i member of) located near to one of the forgers you need to speak to and stashed my stolen goods in a chest there. Although i could not drop the letter i saved my game and went back outside to yield at the guards and then pay my bounty, they carried out their 'procedure' but didnt confiscate my letter. This is not a recommened way at all but if it comes to the stage where you cant get past the guard either then you might as well do what i did! After that part i could carry on with the mission normally following the guide.(I take no credit for the guides in the links)