The very same guy who is almost certainly responsible for the dishonorable nature of the Guild itself.
And guess where this key is : yes in the Snow Veil Sanctum. ) laying around and if you reach the bottom of this ruin, there is an epic boss battle between the ghost of Nocturnal with Karliah against you, if you win this fight you find a note on Karliah's body with the location of the skeleton key and the optional ritual you can perform to gain one of Nocturnal's powers. IF you survive the fight, Karliah runs away to the Twilight Sepulcher but leaving bread crumbs (a key, don't walk in the light.
The player will be greeted by Karliah (don't know how to do this but hey I managed quite a bit, so this wouldn't be so hard to make) and she congratulates you for killing Mercer Frey but she is angry at you at the same time for killing Brynjolf (maybe she had a secret relationship with him and had a shadow child, like in the Game of Thrones), so she makes you an offer, leave the Nightingales alone and (maybe, I don't know how to do this exactly) earn their respect again or be brought down by the shadow legion of the Nightingales (insert epic fight here =P, maybe a level based number of Nightingales ninjas, if your about level 80, maybe 300 ninjas called the Immortals =P). This is the base concept I've come up with : I did work out a base concept but I want to hear what your suggestions are.
As a few people suggested they want to do something about the Nightingales as well. As you all may know, I've made a mod that allows the player to destroy the Thieves Guild and kill/exile Maven Black-Briar.