|Nightingale Armor Set|
With information on what the traitor to the Thieves' Guild, Mercer Frey, is planning to do before leaving Skyrim, it is time for the Dragonborn and the rest of the Guild to put an end to Mercer's activities.
Listen to Karliah
Speaking to Karliah makes it clear that in the current state Brynjolf, Karliah and the Dragonborn will be in no shape to face the powerful Mercer Frey. They need to face him on equal footing as fully-fledged Nightingales.
Meet Karliah at the standing stone
Meet the Brynjolf and Karliah at the Standing Stone, in front of the now unblocked Nightingale Hall. Here Karliah explains about the Nightingales and the Daedric prince of Darkness and patron to thieves: Nocturnal.
Follow Karliah into Nightingale Hall, the meeting place and a refuge for the Nightingales since time immemorial.
Activate the Armor Stone
Equip the Nightingale Armor
The armour has to be warn.
Enter the chamber with three circular pads.
Stand on vacant floor glyph
As soon as the Dragonborn stands on the western circle, Karliah will call forth Nocturnal, asking to take the Oath to become Nightingales, and reinstating Karliah as one again. Nocturnal is not very happy about Karliah's failings to protect the Twilight Sepulcher, but grants the trio their wish.
Speak to Karliah
Karliah then mentions Mercer's true crime: he was able to unlock the Guild Vault without two keys because of what he stole from the Twilight Sepulcher... the Skeleton Key. By doing this, he compromised Guild's ties to Nocturnal and in essence caused their luck to run dry. The key can unlock any door, but not only physical ones, it can also unlock untapped mental powers and thus limitless potential.
On the subject of what to do with the key: Keeping it for the Nightingales is not possible, since their luck would run dry, and luck is something required in their profession. Keeping the key for the Guild would likewise only lead to trouble. So no one should possess the key, something which Karliah applauds.
It is up to them to return the Skeleton Key to its rightful owner.
Speak to Brynjolf
Brynjolf brings up one last important issue, the question of Guild leadership: he and Karliah agree that only due to the Dragonborn's help did Mercer's treachery get exposed. So after dealing with him, all that remains to be done is restore the Guild to its full strength again. Both of them thus feel that the Dragonborn has the potential to replace Mercer as leader of the Thieves' Guild. Brynjolf himself has been around for too long, done questionable things in that time, and does not want that sort of power. And so it is decided that the Dragonborn will become the new Master of the Guild.