Blindsighted

It is the penultimate quest given to the character in the Thieves Guild questline. It is also the quest during which the Skeleton Key is received.

Background

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

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, then move forward, down the hallway. Go past the water and through the gates into the next hall. There is a Centurion Sphere patrolling at the bottom of the stairs. If you are detected, 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 your way around these without making too much noise. You still have enough speed to sneak through them if you time it well. 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, and you will find yourself in a new area where Karliah and Brynjolf are waiting for you. Speak to Karliah, and she will inform you about traps that Mercer may have left.

Locate Mercer Frey
You will find a trap trigger on the bottom left of the doors in front of you. Disarm the trap, or open the doors 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, and three lockpicks. Turn left and head down the stairs, watching for the expert locked gate full of goodies to your left. Karliah will spot Mercer and a short scene will take place. Afterward, 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 through the large area, killing the small groups of enemies that you come across. Once you are at the other end, you will see a tower that 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 the bear traps as you go. Grab the detect life scrolls in the room to your right, as they will come in handy later. In the room with the detect life scrolls, there is writing on the walls from Mercer Frey that says, "One Step Ahead - Mercer". The writing is backwards. Go through the gate on your left (watch out for more bear traps), 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 will be 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 your 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. Watch for the Falmer traps. After the bear traps, you should see some spiders and Falmer across the hall. Once they have been dispatched, head through 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, but 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 past, simply follow the far right-hand side wall and follow it all the way around, into the pen with a small cave at the rear. After the cave, you will find yourself in a larger cave, with two Falmer Skulkers in front of you. Kill them, continue down the ramp, and you will be confronted by several Charus, along with another Falmer Skulker. If you have a high enough sneak skill, it is possible to bypass the Charus and Skulker by sneaking along the golden pipes above their heads. Make your way through the rear of the cave onto a downward hill, making sure to kill the Falmer before dropping down. Head through the gate, then down into Irkngthand Sanctuary. (Note: There are a number of quest-breaking glitches in the next stage.)

Slaying 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 did not take the scrolls, use detect life to reveal him, then stick a Magelight on his head) to see him and take him down.

Strategies
• Mercer uses an invisibility spell, so to see him and take him down: use the detect life scrolls from a previous room/ hit him with Magelight before he becomes invisible/ use the Aura Whisper shout/ look for the red enemy dot on the compass/ look for the telltale ripples in the air and water (This is difficult, ineffective and not recommended but possible). It is also possible to see the edges of the spell covering Mercer if you look close enough. You should be able to kill him somewhat easily without using dectect life or summons.

•Conjured Daedra (or most any other conjure) are effective since they ignore invisibility. Also, conjured Atronachs will make him briefly visible when he is taking fire, ice or lightning damage.

•The Ice Form shout will freeze him in place, allowing for an easy kill.

•Mercer Frey uses weapons with absorb health, so if you have any similar weapons, like the Blade of Woe, that can help counteract this.

•Poison can be used to deal extra damage or add effects that can make the fight substantially easier, especially if you have not prepared for his invisibility, in which case every hit will count.

•A way to instantly kill him is to use daggers with Shrouded Hand Wraps or Shrouded Gloves, then Sneak with the Assassin's Blade perk. If done correctly and you stay hidden, you can deal thirty (30) times your normal damage, enough to instantly kill except (possibly) on very high levels.

•Mercer Frey will often run to the top of the statue. At this point, Unrelenting Force can be used to push him off, resulting in a instant kill.

Once Mercer is defeated, grab both the Skeleton Key and Eyes of the Falmer.

Escape from Irkngthand
Brynjolf and Karliah will jump down to you and the place will start filling up with water, but do not 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 that area and you will find an entrance to 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.

(Having the Waterbreathing spell, enchantment, potion, or playing as an Argonian are useful if the player wishes to explore the chamber once it has sunk. It is easily possible to initially escape the chamber without any of this, though you may miss several items, like Mercer's sword and the second eye of the Falmer.)

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 that 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 while Karliah is too afraid to face Nocturnal after her failure, so you must do this alone. After telling you to return the Skeleton Key, she gives you the Nightingale Bow.
 * 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.
 * Karliah will tell you about the various "Agent" classes that you can receive at the end of the quest, even though you would have no idea they existed.
 * A warning is merited regarding your safety as soon as you leave the cave, as there is often a dragon to deal with near the exit point, so fast travel will not be permitted. It is very rare, but you may have to deal with two dragons simultaneously. One normal dragon, and a uniquely named dragon. Dragons may be more common if you exit at night than during the daytime. If a dragon is present and you do not wish to fight it, it may go away if you go back inside the cave and wait awhile.
 * Very rarely, the dragon will be flying somewhere off to the left as you exit the cave, allowing you to fast travel.
 * It has been confirmed on Xbox 360 that there is a chance no dragons will spawn at all.
 * It has been confirmed on PS3 that there is a chance no dragons will spawn at all, allowing you to fast travel.

Behind the scenes

 * Unless otherwise stated, the information above was obtained from in-game play or from The Elder Scrolls V: Skyrim Creation Kit.