Quick Walkthrough
- Travel to Irkngthand.
- Speak to Karliah.
- Locate Mercer Frey.
- Slay Mercer Frey.
- Retrieve the Skeleton Key.
- Escape from Irkngthand.
- Speak with Karliah.
 Detailed Walkthrough
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 follower, they won't be able to accompany you. It's a winding path to get to the top; you may wish to come during the day so as to see it better.
 Irkngthand Arcanex
Right into the first room, you'll notice an expert-locked chest next to the dead bandits. There is another chest with an adept-level lock at your left, near the crumbled columns. There is also a master-level chest next to the wall by the crumbled columns on your right.
Defeat the Dwarven sphere inside the room with a shallow pool and proceed through the open gate. The next room has three more Dwarven spheres. Before going down the stairs, turn right and follow the path in the upper floor to find a chest. There's another chest close to the wall, on the lower floor.
Continue to the next room and you'll come across several spinning flamethrower traps. Just follow in the same direction they're spinning, and you can make it through without being hurt. You can also use your Become Ethereal shout to get across easily. A couple of Dwarven spiders will attack you on the way.
Enter the corridor and you'll find an apprentice-locked chest straight ahead. At the end of the corridor, there is a Dwarven spider and another chest. Activate the lever and you'll descend to Irkngthand Grand Cavern.
 Irkngthand Grand Cavern
Karliah and Brynjolf will follow you now. Karliah believes Mercer knows that you're following and plans to ambush you down here.
There is a trap on the left side of the door. After deactivating the trap wire, go through the door and continue down the hall. There is a chest behind an expert-locked gate, at your left.
In the large room, 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 once pulling the first lever you must reach and pull the second lever quickly, before the gears activated by the first stop turning. They can be pulled in any order to proceed.
There are two chests on the path to the right lever: one is next to a Falmer tent and the other one is next to the lever itself. Be careful while crossing the chamber as there are multiple traps on the floor.
At the ruins, kill the Falmer and continue through the ramp on the left to find a chest. There is also a chest with an adept-level lock near the collapsed tower, further ahead.
Make your way up the ramps and you'll find a door. Enter it and you will reach a room with a dead Falmer inside. Behind an open gate you'll find some empty chests, scrolls of detect life (which will come in handy when fighting Mercer) and a mirrored message from Mercer. The message reads "One step ahead - Mercer"
In the next room, shoot the Dwarven 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).
Moving ahead, you'll find a door leading to Irkngthand Slave Pens, with a Falmer chest to its right, and a Dwemer chest to its left.
 Irkngthand Slave Pens
The pens are used as a hive to the Falmer. Defeat the Dwarven spider and head up the stairs to find a couple of ingots behind an apprentice-locked gate. As you come back out of the gate, turn right and follow the dead-end path with the glowing mushrooms to find a chest.
Down the stairs, you'll find a torture chamber with two Falmer and a dead thief inside. You can kill both Falmer by sneaking through the grid and activating the trap lever. Proceed through the corridor on the left and Karliah will tell you that you have a choice between sneaking or fighting the Falmer. There is a chest located inside the third Falmer tent on the right. There is also a chest in the tent across from the third on the right.
The next room has a simple pipe maze. Kill the Falmer and head up the ramp on the right to find another chest with an apprentice-level lock. Further down the room you'll face more Falmer and chaurus through the path that leads to Irkngthand Sanctuary. At the bottom of the ramp, turn left toward the tents to find an expert-level locked chest in the eastern-most tent. If you wish to bypass the enemies here, you may also sneak across the overhead pipes. Just before entering the corridor that leads to the Sanctuary, there's a locked master-level chest next to the Falmer door.
 Irkngthand Sanctuary
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 (position yourself on the stairs and let Mercer come to you; for whatever reasons, he doesn't sneak up behind you). Use the Detect Life scrolls (or the Detect Life spell if you have it) to see him; if you have the Aura Whisper shout, this can also be used to reveal his location. If you have the Slow Time shout, use this when he appears in front of you and he can be taken easily if you are leveled high enough. 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 follower, this is where they will be waiting for you. You can give the left eye to Delvin for gold as part of The Litany of Larceny; the right eye can be kept or sold to a merchant.
Talk to Karliah to complete the quest, and she will give you her Nightingale bow.
- In the room after you have to choose the "high road or low road", after descending the ramp with the bear traps you can find an open door on the right side. There are three empty chests of the two different dwemer chest types, a few scrolls to pick up, and a note scratched into the metal door. Written backwards, the message reads "one step ahead -Mercer".
- 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. Using a spell effect such as fire will make him easier to see while invisible
- If asked to stop following you, Karliah and Brynjolf may not automatically start following you again. When you enter the sanctuary, they will appear.
- While the water is filling in Irkngthand, when you touch the water the game may freeze.
- Reloading a previous save from before entering that zone should resolve this.
- Before leaving the sanctuary, be sure to talk to Karliah or both she and Brynjolf may become stuck underwater. At this point you will not be able to talk to her, essentially breaking the quest.
- Use the console commands
prid 58F1Aand then
moveto player. This way you can talk to her and continue the quest.
- Alternatively, you can try swimming and bumping into Karliah repeatedly to knock her into the correct path and out of the water, at which point she will continue normally. This may take a while, so it is a good idea to have a waterbreathing enchanted item with you to prevent drowning (if you are not playing as an Argonian).
- Another option is to shoot her with arrows when she is in sight from the land. She will run up at you. If you have a potion of paralysis, paralyze her when she is on land. Exit and come back.
- Use the console commands
- You may be unable to loot the Skeleton Key and one of the eyes from Mercer's body.
- To solve this, go out to one of the beds at the start of the dungeon, "sleep" in it, and the game will ask "Serve your time in jail?" Click yes and you'll appear outside of the prison you broke out of. Now you can return and loot the corpse.
- Where you enter the room with the statue where Mercer is, you may freeze and the game does nothing after Mercer says "Karliah, When will you ever learn you cannot get a drop on me?"
- Use the console command
setstage TG08B 40to fix this.
- You can also use command
prid 00022651and then
moveto playerto move Mercer to your position and fall down off the ledge, then he will automatically go back to his correct position on top of the statue. He will then continue to cast a spell making the ledge crumble and will speak to you.
- Use the console command
- If you beat Mercer behind the statue's head, he might fall through the ground, making you unable to loot him, thus preventing the quest from advancing. Simply reload to fix this.
- Use the console command
setstage TG08B 50to fix this.
- Use the console command
- Brynjolf or Karliah may leave their posts and walk to Riften as soon as you enter Irkngthand, rendering the quest non-completable. Their dialogue may say that they are still in Irkngthand. You can find them on the path to Riften from Irkngthand by following your quest marker. You will be able to then start the quest with Karliah, although a short run back to Irkngthand will be required. Make sure to keep Brynjolf and Karliah close during your advance back up to Irkngthand as not zoning into Irkngthand means that you are not guaranteed any followers.
- To fix this, simply go through the dungeon yourself, and they will appear in the Sanctuary once you get there.
- The game will freeze or lag in the middle of Irkngthand.
- One known fix is to restart the console and reload your last save.
- If you speak to Karliah again after escaping from the water and after the dialogue she initiates, the game will display dialogue options that should only become available at the end of the quest Darkness Returns.
- If you enter the Sanctuary while sneaking, Karliah and Brynjolf may not enter and Mercer will be unable to climb down the statue.
- In the Irkngthand Grand Cavern, Brynjolf will ask you to choose the between the high or low road. You may see a Falmer stuck inside of one of the tents. If you kill the Falmer in the tent and enter the tent to loot, you may become stuck as well.
- This bug is fixed by version 2.0.2 of the Unofficial Skyrim Patch.
 Quest Stages
- The following empty quest stages were omitted from the table: 0, 1, 32, 33, 35, 37, 41, 47.
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