Blindsighted

Walkthrough
Asked if the Dragonborn may return to Nightingale Hall, Karliah says "yes", since they are Nightingales now, and to consider the hall home. It will offer many things that will help on future endeavours and contain a wealth of information. But only after the Skeleton Key has been restored to the Twilight Sepulcher. Karliah will make the hall her home as well.

And since the key has not yet been returned the trio cannot yet benefit from Nocturnal's gifts. So far merely the Oath was transacted with Nocturnal. But holding her favour should be enough of an edge to confront Mercer Frey.

Travel to Irkngthand
Irkngthand is another location that a follower cannot enter, so the Dragonborn must face some of the dangers alone, at least in the Irkngthand Arcanex.

Speak to Karliah
Just before entering the Irkngthand Grand Cavern the Dragoborn meets up with Karliah and Brynjolf again. The bandits in the first chamber were killed by Mercer. Karliah warns of further insidious traps. Brynjolf points out how Mercer is using the ruins and its constructs to his advantage, to wear down the trio. As research in the matter yields, a huge statue with gemmed eyes, flawlessly cut as large as a man's head, exist in this complex, Mercer's goal.

A slightly dubious bottle of Black-Briar Reserve can then be found on a stone table with the "Guild scratchmark".

Soon after Mercer can be seen through metal grating, taking down some Falmer. But he cannot be reached, so the trio has to navigate the ruins, into the Irkngthand Slave Pens, where an unlucky Thief left behind a note with the Thief's Last Words.

Locate Mercer Frey
Finally entering the Irkngthand Sanctuary they have located Mercer Frey, as he still is removing the Eyes of the Falmer stones from a large stone Falmer statue.

A short dialogue with Mercer lets the Dragonborn clarify why they are here for the Skeleton Key: because of Nocturnal's guidance, because it's personal, or because of the Eyes. As that may be...

Slay Mercer Frey
Mercer then immediately attacks. After a short battle the cave starts to cave in and the water level starts to rise.

Retrieve the Skeleton Key
Looting Mercer Frey yields not only the Skeleton Key but also the Left Eye of the Falmer and the Right Eye of the Falmer; a total of 10330 value on his body.

Escape from Irkngthand
With the main chamber door blocked and the water rising in the chamber, swim towards the rear and up to find an exit to the Bronze Water Cave. (It is located near the top of the Falmer Statue, and the exit to the cave is in the ceiling. You must first wait for the water to completely rise, then swim through the exit on the ceiling.)

Speak to Karliah
Karliah is very glad that her 25 years of exile are over, and that the Skeleton Key has finally been retrieved and rewards the Dragonborn with a bow very special to her: Nightingale Bow.

The final leg, returning the key to the Twilight Sepulcher will need to be done by the Dragonborn solo, since Brynjolf has to look out for the Guild in Riften, and Karliah just cannot bring herself to enter the place, after having failed Nocturnal all those years ago. Continued in Darkness Returns.

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