Blindsighted

It is the second to last quest given to the character in the Thieves Guild questline. It is also the quest upon which the Skeleton Key is received.

Background

 * Key NPCs: Karliah, Brynjolf, Mercer Frey
 * Enemies: Bandits, Falmer, Centurion Spheres, Dwarven Spiders, Dwarven Centurion.

Objectives

 * 1) Travel to Irkngthand
 * 2) Speak to Karliah
 * 3) Locate Mercer Frey
 * 4) Slay Mercer Frey
 * 5) Retreive the Skeleton Key (Skyrim)
 * 6) Escape from Irkngthand
 * 7) Speak to Karliah

Travel to Irkngthand
Upon arriving at Irkngthand, be wary of bandits who have set up camp here. You will need to fight your way up to the top to gain entry to Irkngthand. ''Note: If you have not started this quest, you cannot gain full entry to this dungeon. (Dungeon Lock Outs) Note: any follower you have with you cannot enter the ruins whilst you have this quest, however they will be waiting for you in Bronze Water Cove.''
 * Journal Entry: I'm on my way to Irkngthand, a vast dwarven ruin that supposedly contains the legendary "Eyes of the Falmer." Karliah and Brynjolf are waiting for me within. Together we hope to stop Mercer Frey from stealing the Eyes and escaping Skyrim with the Skeleton Key of Nocturnal.

Speak to Karliah

 * Journal Entry: I've entered Irkngthand, a vast dwarven ruin that supposedly contains the legendary "Eyes of the Falmer." Karliah and Brynjolf are waiting for me within. Together we hope to stop Mercer Frey from stealing the Eyes and escaping from Skyrim with the Skeleton Key of Nocturnal.

Upon entering Irkngthand, make your way across the hall past the fire and bodies and up the stairs. Make a right and then make your way along the hallway. Go past the water and through the gates into the next hall. There is a sphere at the bottom of the stairs patrolling, as well as other spheres that will come out the holes in the walls as soon as you are detected, so dispatch them as quietly as possible (One way is to use a sneak bow attack and take them out one by one waiting in the shadows and letting your sneak say "Hidden" between hits) In the next room, rotating flame towers will activate. You need to make you way around these without making too much noise, you still have enough speed to sneak through them, just time it well enough. At the final flame tower, make your way all the way around it and then open the gate to the right of it. Make your way up the stairs until you reach the master locked gate. Opposite the gate is a lever, pull it. You will find yourself in a new area where Karliah and Brynjolf are waiting for you. Speak to Karliah, and she informs you about traps that Mercer may have left.

Locate Mercer Frey
Open the doors in front of you and watch for the flying mace ball. Turn left and head down the stairs (watch for the expert locked gate full of goodies to your left as well). Karliah will spot Mercer and small scene will take place. Afterwards, follow the path round and open the door. Head down until you reach the next hall. A Falmer Gloomlurker will attack, between the three of you, you should quickly dispatch it. Head over the mound to your left towards the lever and pull it. Across the hall, you will spot another lever that you will need to also pull in order to open the door. You can either do one of two things: Once they are taken care of, pull the other lever and the door will open. If you take to long, you will notice that you have to pull the other lever again as it is set on a time limit, just pull one and immediately sprint to the other until you get it. Once the door is open, head down the stairs, watching for the spinning blades in the ground and head through the door. You will find yourself in a huge hall area, make your way along killing the Falmer Gloomlurker's as you go. Don't panic when the ground shakes. Once you are at the other end, you will see a tower has collapsed and you must make your way up the spiral ramp and through the door at the top. Go down the stairs and watch for the bear traps as you go. Grab the detect life scrolls in the room to your right, they will come in handy later. Go through the gate on your left and then through the door at the end of the corridor and you will find yourself, once again, in a large hall. Brynjolf will tell you that you can sneak around or take on the Dwarven Centurion. Since there are a lot of Falmer in this room, you can jump off the little bridge to the right and on your left there is a button that activates the Centurion. This will soften up the Falmer and they should kill the Centurion for you.
 * Journal Entry: Karliah and Brynjolf have joined me in my exploration of Irkngthand, a vast Dwarven ruin that supposedly contains the legendary "Eyes of the Falmer." Together we hope to stop Mercer Frey from stealing the Eyes and escaping Skyrim with the Skeleton Key of Nocturnal.
 * 1) Dispatch the other two Falmer Gloomlurkers below using arrows
 * 2) Sneak over to the other side of the hall (towards the lever) and find the expert locked gate, unlock it and use the explosive arrows to take out the Falmer Gloomlurkers (use Throw Voice to get them into position if you have it)

Regardless of choice, make your way to the cage and then head up the stairs. You should see the path lead up and around to where you want to go, watching for the Falmer traps. After the bear traps you should see some spiders and Falmer across the hall. Once they have been dispatched, head into the door leading to the Irkngthand Slave Pens. Make your way down the stairs and quickly kill the Dwemer Spider, then head left and continue down the stairs. Activate the lever and a spinning blade will take out the two Falmer down below, but remember to turn it off before continuing. Run down the hall and be careful of the three flame pressure plates near the corner. Once in the next hall, Karliah will tell you there are a lot of Falmer in the next room and to sneak or take them on. If you want to sneak, simply stick the far right hand side walls and follow it all the way around and into the pen where there is a small cave at the back. After the cave you will find yourself in a larger cave, with two Falmer Skulkers in front of you. Kill them and continue down the ramp, and you will be confronted with several Charus, along with another Falmer Skulker. Make your way through the back of the cave into a downwards hill, making sure to kill the Falmer before dropping down. Head through the gate and then down into Irkngthand Sanctuary.

Mercer will be removing the eyes of the statue as you enter. He spots your group and makes the ledge you are standing on collapse. Before the battle begins, he will talk to you for a bit about the Thieves Guild and honor. Mercer uses an invisibility spell, so use detect life scrolls (if you didn't get the scrolls, use detect life to reveal him then stick a magelight on his head.) to see him and take him down. Another good tactic that usually ends in an instant kill is to climb to the top of the statue and use Unrelenting Force to fling him off. Once he's down, grab both the Skeleton Key and Eyes of the Falmer.

If you have the Aura Whisper shout you can use it to find him or use the Ice Form shout to freeze him in place and kill him. If he turns invisible (at the very bottom floor where it is submerged in water) there will be rifts in the currents of the water that reveal his location.

Escape from Irkngthand
Brynjolf and Kalirah will jump down to you and the place will start filling up with water, but don't panic. Once the water is high enough, a piece of the statue's head will fall off and open an area behind it. Head into the area and sprint until you find yourself in Bronze Water Cove.
 * Journal Entry: Mercer Frey is dead and I've recovered the Skeleton Key of Nocturnal from his corpse. I need to escape Irkngthand before the place collapses around me.

Speak to Karliah
Upon speaking to Karliah she will tell you that returning the key will not be easy. The inner sanctum has been locked due to the fact the Nightingales failed to protect the key in the first place and the only way through is via the Pilgrim's Path. Brynjolf heads back the guild whilst Karliah is too afraid to face Nocturnal after her failure, so you must do this alone.
 * Journal Entry: Safe from the dangers below, Karliah, Brynjolf and I finally have a moment to breathe. Mercer Frey is dead, the Skeleton Key of Nocturnal is in my possession and, most importantly, the Nightingales have survived.

Bugs

 * It is strongly recommeded to save your game before entering the final room with Mercer due to the large amount of bugs.
 * When you run past all of the enemies in the cavern before you see Mercer, and then enter, Karliah will tell Brynjolf to watch the door, and the player will get stuck. If you save before entering the room, wait a little, and then enter, there will most likely be no problems, and the cut scene will go as planned. (If this doesn't work, open the console and type "setstage TG08B 40").
 * A small glitch can occur during the stage of the quest after getting the Eyes of the Falmer. Mercer will make his way for the stairs, but get stuck and begin glitching up and down the wall, causing you to be frozen in the cutscene. Mercer eventually fixes himself.
 * There is a possible major "quest-breaking" glitch. When you swim out of the cavern after it fills with water, both Karliah and Brynjolf can get stuck in the flooded cavern. To end this quest you have to talk to Karliah, but you cannot talk to her when she is underwater. Reloading a previous gamesave can solve this problem, or on PC you can use the console and type "player.placeatme 0001B07F" for Karliah and "player.placeatme 0001B07D" for Brynjolf. It is also possible to fix this by entering the flooded room again, and then coming back out (fix on Xbox 360).
 * Unfortunately, if you leave too many times, Karliah and Brynjolf will be walking along the bottom of the cave while its flooded, as if there was no water in the cave. Solutions are currently unknown.
 * I had the same problem of Karliah and Brynjolf not leaving the room after the flooding. What I did to fix this was after the flooding began and beforthe two of them got off the ledge next to the door and started to swim is I shot them both with my fire enchanted daedric bow. After that, they followed me out of the cave and all was well. (fix for Xbox 360)
 * Another quest-breaking glitch is that Mercer may not appear in the cavern at all. To solve this, save before you enter the cavern, then 'player.moveto 22651' and click on Mercer with the console open. Then, reload your save, enter the cavern, and 'enableplayercontrols'. Jump off the ledge, then type 'moveto player' in the console. Mercer should float his way up to the head of the statue and collapse the ledge, then run down to face you, allowing you to resume the quest as normal. It's possible that turning collision off may break this.
 * Yet another quest-breaking glitch is that after leaving Irkngthand and returning, the quest marker is way off and neither Brynjolf nor Karliah are there. Continuing on as if they were there does advance your Objectives, but I assume that you can't finish it in the end. If you exit Irkngthand you can see Karliah outside, apparently trying to get inside. Any attempt at communication states she is busy. No idea to a fix yet.
 * More quest-breaking glitches, as in some cases Mercer won't have the Left Eye of Falmer and the Skeleton Key on him. Thus, not being able to escape and getting stuck in the chamber.
 * Another bug encountered during the "locate Mercer Frey" section of this quest involves Brynjolf getting stuck, making him unable to follow you. If you go back and find him, you'll find that he will crouch and draw his weapon as you do, but he will no longer follow you. I have no idea how to fix this, and the only way that I was able to get him to follow me again was to restart from the "speak to Karliah" part of the quest.
 * Yet another quest-breaking bug, just before the final battle with Mercer, upon entering the room where he removes the eye from the statue, the player is told that "[his] follower can't follow [him] here" then Karliah tells the Dragonborn to sneak down the ledge, and Mercer turns around saying "Karliah, when will you learn you can't get the drop on me?" and finally the player gets stuck. There are no possible moves, no interaction, 3rd person view is unavailable, can't bring up the tab menu. On PC, the workaround is to open the console and fiddle with the tcp command. Try hitting the console key when the map loads, enter "tcl", exit console, see if the cutscene goes further, and if it does and gets stuck again, deactivate the tcl command.
 * Brynjolf and Karliah can end up fighting one another if they're caught in the crossfire of the player's spells. Save often so reloading without too much lost progress is possible when this happens.
 * Brynjolf and Karliah might end up fighting one another after the cavern is filled with water, making it impossible to interact with Karliah to advance the quest.
 * PC Fix: Use the console and type prid 58f1a to select Karliah, then moveto player Repeat for Brynjolf using prid 20545 This will bring both NPCs to the surface where Brynjolf will move to the exit of the cavern while Karliah will interact with the player to complete the quest. Using SetStage will complete the quest and the player will receive the bow; however, both NPCs will remain underwater.
 * When passing the flame spinning room, Karliah and Brynjolf may leave the building and wander off into the woods. To fix this, just carry on the quest without them and they will appear in the end room with Mercer.
 * In the room where the giant statue crumbles, there will be a severe framerate lag after entering the second door on the far side of the room. Making a save and loading it can usually solve the problem.
 * Another game-breaking bug (only PC confirmed): Karliah's dialogue after killing Mercer may be bugged, only completing the quest but not activating Darkness Returns.
 * Another major bug (only PC confirmed): Karliah might disappear from the cavern upon meeting Mercer, therefore rendering the completion of the quest impossible. Karliah disappears completely from the room and is not found e.g. by utilizing detect life spell. Opening the console and typing "player.placeatme 0001B07F" seems to fix this although there might be ramifications which are not currently known.