Blindsighted

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

Background
''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 from Skyrim with the Skeleton Key of Nocturnal.''

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
''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.''

Upon arriving at Irkngthand, be wary of bandits who have set up camp here. It will be necessary to fight a way up to the top to gain entry to Irkngthand. ''Note: If this quest has not been started, full entry to this dungeon cannot be attained. (Dungeon Lock Outs) Note: any follower with the Dragonborn cannot enter the ruins whilst this quest is active; they will be waiting for the Dragonborn in Bronze Water Cave.''

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, go across the hall, grab the loot in the chest by the fire and bodies and go 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 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.) It is also possible to sneak past the Spheres without being noticed. Go to the left of the gate and jump over the fence. Go into stealth and sneak past the one sphere that deployed. In the next room, rotating flame towers will activate. Move around these without making too much noise. The towers are slow enough to sneak through them if timed well. At the final flame tower, go around it to the left and then open the gate to the right of it. Move up the stairs until the master locked gate is reached. In front of the Dragonborn there will be a ramp with a Dwemer Spider on top. Kill it and hop over the rubble pile to find a hidden chest. Opposite the master locked gate is a lever. Pull it, and it will open a new area where Karliah and Brynjolf are waiting. Speak to Karliah, and she will explain about traps that Mercer may have left.

Locate Mercer Frey
''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.''

There will be a trap trigger on the bottom left of the doors in front. 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 loot to the left. Karliah will spot Mercer and a short scene will take place. Afterward, follow the path round and open the door. Head down until the next hall is reached.

Upon entering the next hall, turn right and collect the falmer chest on the right side next to the falmer hut. Then turn arround 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. Once at the other end, a collapsed tower can be seen, and it is necessary to go up the spiral ramp and through the door at the top. Before leaving this room make sure to stop and collect the goodies that are left in the Falmer chest. The chest is located next to a Falmer tent to the left on the way to the ramp out.

Go down the stairs, carefully avoiding the bear traps. Grab the two Detect Life scrolls in the room to the 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 to the left (watch out for more bear traps), then through the door at the end of the corridor, which leads to a large hall. Brynjolf will explain that the options are to either sneak around or take on the Dwarven Centurion. Since there are a lot of Falmer in this room, it is possible to jump off the little bridge to the right, and on the left there will be a button that activates the Centurion. This will soften up the Falmer, and they should kill the Centurion.

Regardless of the choice, continue to the cage, and then head up the stairs. There should be a path that leads up and around to the next destination. Watch for the Falmer traps. After the bear traps, there should be some spiders and Falmer across the hall. Once they have been dispatched, collect the loot in the chest next to the Falmer tent at the bottom of the stairs and head through the door leading to the Irkngthand Slave Pens. Go down the stairs and quicly 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 say that there are a lot of Falmer in the next room, and will give the option to either sneak or take them on. If it seems easier 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. There is a chest next to the first fire pit in a Falmer tent and another one in a tent directly across from the first. After the cave, the party will be in a larger cave, with two Falmer Skulkers in front. Continue down the ramp, and there will be several Chaurus, along with another Falmer Skulker. If the Sneak skill is high enough, it is possible to bypass the Chaurus and Skulker by sneaking along the golden pipes above their heads. Continue 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 the party enters. He spots the group and makes the ledge the party is standing on collapse. Before the battle begins, he will speak shortly about the Thieves Guild and honor. Mercer uses an invisibility spell, so use Detect Life scrolls (if the scrolls were not taken, use Detect Life to reveal him, then stick a Magelight on him) to see him and attack.

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

Escape from Irkngthand
''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.''

Brynjolf and Karliah will jump down and the place will start filling up with water. Once the water is high enough, a piece above the statue's head will fall off and open an area above it. Head into that area which, will lead to an entrance to Bronze Water Cave.

Speak to Karliah
''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.''

Upon speaking to Karliah, she will explain 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 the Dragonborn must do this alone. After telling the Dragonborn to return the Skeleton Key, she hands over the Nightingale Bow.

Karliah will tell the Dragonborn about the various 'Agent' aspects that can be assumed at the end of the quest.

Trivia

 * At the end of the mission, Karliah's dialogue is wrong.
 * This is the only Daedric quest that does NOT award the Dragonborn with a personal artifact. The received Skeleton Key counts as a quest item for the next quest, so it is impossible to keep it unless one chooses to ignore the Thieves Guild questline.
 * Also, the Skeleton Key does not count towards the Oblivion Walker achievement.

Bugs
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