Allows to free living Nightingales from the Oath to Nocturnal for Dragon Souls. Nocturnal sells a spell to summon Mercer Frey.

If you feel uncomfortable knowing that your soul is being sworn to serve a Daedric Lord, you can change it. After Skeleton Key related quests are done, Nocturnal can be summoned in the Twilight Sepulcher. Offer her Dragon Souls, and she will agree to free you from the Oath. You won't be a Nightingale any more. The entrance to the Twilight Sepulcher will be closed, you will lose your Nightingale armor and abilities. But freedom is better.

Brynjolf and Karliah can be released too. If you talk to them before have bought your freedom from Nocturnal, they have appropriate dialogues. Brynjolf isn't happy about all this in any case, so once you've talked to him - Nocturnal will accept payment for his soul too.
Karliah is a Nightingale from the very beginning and wishes to stay one. So normally you are unable to discuss Karliah with Nocturnal. But if her relationship rank to you is 4 (which is lover) or/and you two are married she won't be pleased with her destiny, and also can be freed from the Oath.

For Dragon Souls you can buy a spell to summon Mercer Frey as your ally. He fights for you and his only presence improves your skills. But randomly he will go hostile (you killed one another, after all). What you gain while he is with you:
. better sneaking
. better pickpocketing
. lockpicks are unbreakable
. noise from armor is reduced
. walking and running doesn't affect detection
. can pickpocket equipped items and weapons
. you don't trigger pressure plates, but he still does, :) and :(

I made it for my main playthrough, but maybe it will be useful for some of you. I apologize for mistakes in English, bet they are. Lipsync for Brynjolf is awful, I will remake it. In the code there are some places which I will remake too (it works, but it pains to look at how it was made).

On Steam http://steamcommunity.com/sharedfiles/filedetails/?id=107876561