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) Retrieve the Skeleton Key
 * 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; they will be waiting for you in Bronze Water Cave.''
 * 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.

Once inside make your way across the hall past the fire and bodies and up the stairs. Make a right and then move forward down the hallway. Go past the water and through the gates into the next hall. There is a Centurion Sphere at the bottom of the stairs patrolling and if you are detected then more will come out the holes in the walls, 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 trap. On the little table is a bottle of Black-Briar Reserve, presumably a calling card from Mercer Frey. Turn left and head down the stairs watch for the expert locked gate full of goodies to your left. 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. W hen you enter the next hall, turn left and head over a small mound to find the lever for the gate below. There is a second lever on the right side across the large set of facing stairs that must be activated quickly after the first or it will reset and the door will not open. Once the door is open head down the stairs, watching for the spinning blades in the ground, and head through the door. move along through the large area killing the small groups of enemies that you will come across. 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 carefully avoiding bear traps as you go. Grab the detect life scrolls in the room to your right, they will come in handy later. In the room with the detect life scrolls there is writing from the walls from Mercer Frey that says,"One Step Ahead." The writing is backwards. 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.

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. (Note: There are a number of quest-breaking glitches in the next stage.)

Statue_of_the_falmer.jpg Mercer Frey
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. Conjured Daedra are also effective since they ignore invisibility. 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. You can also look at your compass, and locate him by looking for the red blip.

It is also simple to use Unrelenting Force on him at the very top throwing him off killing him instantly at the bottom.

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 above it. Head into the area and sprint until you find yourself in Bronze Water Cave.
 * 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 to 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.
 * One interesting thing is that Karliah will tell you about the various "Agent" classes that you can recieve at the end of the quest, even though you would have no idea they existed...not a bug or anything, just kinda interesting.

Bugs
It is STRONGLY recommended to save the game before entering the final room with Mercer (Irkngthand Sanctuary) due to the significant amount of quest-breaking issues at the end of this quest.


 * Running past all of the enemies in the cavern before Mercer is seen and entering may cause the player to get stuck. If the player saves before entering the room, waits a little, and then enters, the cut scene will most likely go as planned. (If this doesn't work, open the console and type "setstage TG08B 40").
 * After getting the Eyes of the Falmer, Mercer will make his way for the stairs, but may get stuck and begin glitching up and down the wall, causing the player to be briefly frozen in the cutscene until Mercer becomes unstuck -should that fail to happen, pausing and unpausing may fix it.
 * Quest-breaking glitch: When the player swims out of the cavern after it fills with water, both Karliah and Brynjolf can get stuck in the flooded cavern, making the player unable to complete the quest by takling to Karliah. Reloading a previous save may resolve the issue. Using the commands "player.placeatme 0001B07F" for Karliah and "player.placeatme 0001B07D" for Brynjolf fixes this. It is also possible to fix this by entering the flooded room again, and then coming back out.
 * If the player leaves and re-enters too many times, Karliah and Brynjolf may occasionally continue walking along the bottom of the flooded cave as if there were no water.
 * Another fix would be to shoot shoot both Karliah and Brynjolf with a bow before they leave the ledge by the door.
 * Quest-breaking glitch: Mercer may not appear in the cavern at all. To fix this, one can save before entering the cavern, then use the command "player.moveto 22651" and click on Mercer with the console open. After reloading the save, and re-entering the cavern, the "enableplayercontrols" command should be used, followed by a jump off the ledge, and finally the "moveto player" command.
 * Quest-breaking glitch: After leaving Irkngthand and returning, the quest marker is completely inaccurate and neither Brynjolf nor Karliah are there. Continuing on as if they were there advances the quest objectives but not the quest. Karliah can be seen outside of Irkngthand, but any attempt to communicate with her states she is busy.
 * Quest-breaking glitch: Occasionally, Mercer will not have the Left Eye of Falmer and the Skeleton Key in his inventory.
 * Another bug encountered during the "locate Mercer Frey" section of this quest involves Brynjolf getting stuck in his position.
 * Hitting him once with Unrelenting Force can fix this. He can also be attacked in order to move him.
 * Quest-breaking glitch: Just before the final battle with Mercer, upon entering the room where he removes the eye from the statue, the player may become stuck, unable to move, interact or open up the tab menu. This may be fixed by entering "tcl", using the console, when the map loads, then deactivating it if the issue isn't fixed yet.
 * Brynjolf and Karliah can end up fighting one another if they're caught in the crossfire of the player's spells, or once the cavern is filled with water, making it impossible to advance the quest.
 * 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. Carring on the quest without them will make them reappear in the end room.
 * 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. Reloading a save usually fixes this.
 * Game-breaking bug: Karliah's dialogue after killing Mercer may be bugged, only completing the quest but not activating Darkness Returns.
 * Karliah might disappear from the cavern upon meeting Mercer, rendering the completion of the quest impossible. Opening the console and typing "player.placeatme 0001B07F" fixes this.
 * Occasionally, the game performance suffers in the room where Mercer is first seen through the bars.
 * Attempting a sneak attack kill on Mercer Frey will trigger the death animation. but his corpse might falls through the floor and becomes unlootable.
 * When entering a falmer tent, the player may get stucked inside, unable to exit.