- Travel to Irkngthand.
- Speak to Karliah.
- Locate Mercer Frey.
- Slay Mercer Frey.
- Retrieve the Skeleton Key.
- Escape from Irkngthand.
- Speak with Karliah.
After becoming a Nightingale, speak to Brynjolf. You learn that he and Karliah decided that you should be the one to replace Mercer once you deal with him. After you accept, he tells you to meet them at Irkngthand to stop Mercer Frey from stealing the Eyes of the Falmer and escaping with the Skeleton Key of Nocturnal.
There are some bandits camped outside Irkngthand, but they’re okay to kill or sneak past. The entrance is at the very top of the ruins. If you have a companion, he or she cannot follow you inside.
Once inside, it is recommended you sneak everywhere because there are irises on the walls which will drop out Dwemer Sphere Guardians (Masters if the player is at a high level) that you’ll have to fight. After getting past the first rooms, you’ll come to a room with several spinning flamethrower traps. Just follow in the same direction they’re spinning, and you can make it through without being hurt. After that, you will come to a Dwemer lift to take you to the level where Karliah and Brynjolf wait.
Karliah and Brynjolf will follow you now. Karliah believes Mercer knows that you’re following and plans to ambush you down here. Go through the door and continue down the hall. You will see a scene of Mercer sneak attacking a Falmer, but you’re unable to get to him. To open the door in the next room, you have to activate the levers on either side of the room overlooking the camp – these levers activate gears to either side of the gate, so you must reach the second lever before the gears activated by the first stop turning.
Brynjolf will tell you that you have a choice between sneaking or fighting, but both he and Karliah will react to sounds from Falmer. (Brynjolf even triggered the Dwemer Centurion standing guard in one area.) Both of them are essential and cannot be killed, but the hazards can reduce them to non-fighting status and leave you facing a pack of angry Falmer. Shoot the Dwemer Centurion with an arrow while still standing on the steps, the Falmer will then fight and destroy him (it can also be activated via a button located on the side of the steps, if you drop down on the right-hand side).
In the sanctuary, you’ll see a short scene of Mercer prying out the eyes of the Falmer statue. Mercer will then collapse the ledge you’re standing on, and after a short conversation, you will have to fight him. During the fight, he will use his Agent of Subterfuge power on Brynjolf, causing him to attack Karliah. This will keep Brynjolf and Karliah out of the fight. Mercer will turn invisible occasionally during the fight. One easy way to end the battle quickly and effortlessly is to climb up to the ledge behind the statue. When Mercer comes to you, use Unrelenting Force to blow him off the ledge, a fall from this height will kill him instantly. Once Mercer dies, a pipe at the top of the room will burst, flooding the room.
Take the Skeleton Key and the Eyes from Mercer’s body, and wait for the water to rise. Once it gets high enough, a hole will appear in the roof above the statue’s head. Run out through the short passage to escape. This takes you to Bronze Water Cave, if you had a companion, this is where he or she will be waiting for you. You can give one of the eyes to Delvin for gold as part of The Litany of Larceny.
Talk to Karliah to complete the quest, and she will give you her bow.