Skyrim:Trinity Restored

Quick Walkthrough
#Speak with Brynjolf and Karliah at the Thieves Guild.
#Meet Karliah at The Shadow Stone just outside Riften.
#Enter Nightingale Hall and follow Karliah's instructions.
#Equip the Nightingale Armor.
#Proceed further into the cave and follow Karliah's instructions.
#Speak to Brynjolf about the leadership of the Guild.

Detailed Walkthrough
Plans Revealed
Return to the Thieves Guild after your search at Riftweald Manor to show Brynjolf Mercer's plans for stealing the Eyes of the Falmer. Brynjolf will explain the significance of your findings; Mercer plans one more heist before disappearing for good. The only hope of catching him is to intercept him at the location. Brynjolf informs you that Karliah has requested to speak with both of you.

A Secret Order
When you speak to Karliah, she reveals to you and Brynjolf that Mercer Frey was a Nightingale, an agent of Nocturnal.  Because of this, Karliah is concerned about being at full strength and on equal footing before pursuing Mercer. She asks you and Brynjolf to meet her at The Shadow Stone outside of Riften. 

The Nightingale Edge
Here you will find yourself at the entrance to Nightingale Hall wherein is the edge over Mercer that Karliah was alluding to. Proceed inside, but don't get too far ahead as Karliah begins telling you more about the Nightingales and their role. She will later remark that this was the Nightingales' base, and that she will return here once done with Mercer. After crossing a bridge, you can find a copy of the Pickpocket skill book Purloined Shadows to the right.

Your first stop is the armor chamber. Activate the stone Karliah indicates and the Nightingale Armor is placed in your inventory. You must equip it in order to continue. Once you have donned the armor, Karliah will discuss the next step: your sworn oath to Nocturnal. Follow Karliah further into the cave past side passages leading to sleeping quarters where you can find a copy of the Sneak skill book The Red Kitchen Reader and living spaces.

Trinity Restored
thumb|right|The first step towards your induction.
Once you have agreed to Nocturnal's conditions on becoming a Nightingale, Karliah will operate the pull chain for the gate leading into oath taking chamber. You will be directed to stand on a specific platform. If you have a follower with you, their presence does not affect the outcome in any way. Karliah summons Nocturnal and asks to resume her role as a Nightingale as well as induct you and Brynjolf into their ranks. Nocturnal agrees. 

Now that you are officially inducted into the Nightingales, Karliah will explain that Mercer got into the vault by using the Skeleton Key which he stole from the Twilight Sepulcher; a grievous insult to Nocturnal which threatens the very foundations of the Nightingales. The key needs to be returned to the Sepulcher to set things right and return the uncanny luck of the Thieves Guild. 

This is a big day for you as now Brynjolf has something to add. With Mercer disgraced, the issue of the leadership of the Thieves Guild is at hand. Brynjolf states that he and Karliah think you would be the right person for the job, once you've proven yourself by dealing with Mercer and completing all of the Thieves Guild additional jobs. From here, leave for Irkngthand to begin Blindsighted.

Bugs
 
Reloading from an earlier save may fix this.
  Use the console command   to have her start speaking, regardless.
  If Karliah or Brynjolf are stuck (won't move, don't complete actions), the console command   will sometimes force them to resume their usual scripted behavior.
 
Push him through the cave.
 You can use the console by clicking on him (or typing  prid 20545 ), and then typing  moveto player  to teleport him to you. Make sure he's near you when you don the armor when Karliah is about to ask you to take the oath, which should fix his AI (confirmed by his comment on his new outfit).
 If only you and Karliah make it to the stones before taking the oath and Brynjolf is still walking away, use console command  . This starts the cinematic sequence without Brynjolf, who will join the show slightly later.
 
Karliah will sometimes go back to the Frozen Hearth where you have met to decrypt Gallus' journal. It is related to the bug in the quest Hard Answers where the stages will not update. Redo Hard Answers step by step, and the quest Trinity Restored will reactivate.
 You can use the console and type   and go to the cellar in the Frozen Hearth to talk again to Enthir and Karliah. After this Karliah and Brynjolf will be at the entrance of the Nightingale Hall. (You can further check if this is the problem by typing   in the console. If the Stage is smaller than 200 then this is the cause of Karliah's disappearance). If setting the stage to 60 doesn't work, enter   to complete the quest Hard Answers. Karliah will then begin her long trek to Nightingale Hall. Fast travel to the hall, wait for 24 hours and she will show up.
 Alternatively, type in the console   (for Brynjolf) or   (for Karliah), followed by the command  . Use the latter command as many times as necessary to lead the selected NPC to the stones; you may need to activate the stone multiple times to get Brynjolf and/or Karliah to equip their Nightingale armors. You may still be required to use the command as you continue further down the halls. Do not use the   commands unless either NPC has completely disappeared from the game (i.e. not stuck or lost), otherwise you will create a clone of the NPC which may cause undesirable results in-game.
 
After Brynjolf accepts the terms of becoming a Nightingale, Karliah will ask you if you accept. Accept the terms, Karliah will then turn and head to the gate. Immediately interrupt her and ask her any question. This lets Brynjolf move ahead of her and fixes this bug. In some cases you might need to interrupt Brynjolf rather than Karliah and let her get to the gate first. This can be achieved by standing in his way.
Attacking Karliah may cause her to turn around and open the gate.
Exiting the conversation with her will allow the stage to reset itself, and may fix this.
Since the bug seems to involve the conversation with Brynjolf, you can sometimes circumvent the bug by talking to Brynjolf continually until Karliah opens the gate.
Allow the cell to completely reset and quickly run to the gate.
You can also fix Karliah from walking away if you tell her that you don't understand the full meaning of being a Nightingale. She will ask you again and then if you say you are ready, you can enter the room to finish the ritual.
Before accepting Nocturnal's conditions from Karliah, force her to and keep her directly at the gate using Unrelenting Force and Paralyze. Accept the conditions and keep her at the gate until Brynjolf joins you both. She may then pull the chain and progress the quest.
 Open the console, click on the gate and use the command  , then  . You can now see the cinematic involving Nocturnal. As with all console commands, exercise extreme caution.
 Karliah may be going to the exit instead of opening the door to the Nightingale contract. To repair this, use   on the door, and use   on Karliah and   to fix her before using    to teleport her to you. After that, use   and they may walk to their own spots.
 The same can be done as above only instead of disabling the door, it is possible to run   on the door (which simulates Karliah pulling the chain instead of removing the door from the game).
 Also, you can attack Karliah while she is talking to Brynjolf and use relenting force to move her to where the chain is. After, use Emperor's Voice to calm her down and then talk to her. She will then pull the chain and you can proceed. If you are not an imperial and do not have this power, you can use the command   to change it to imperial and then change it back to your original race. 
  If none of the above works, run to the gate while Karliah talks to Brynjolf. After they are done talking, open the console and execute the following commands:   and  . This should teleport Karliah to you. If she doesn't start a conversation with you just talk to her. After you accept the oath she should now open the gate and proceed inside the chamber
  The following solution worked on multiple tries. After accepting the oath, run up to the chain by the bars and open up the console and execute the following commands  . Then execute  . That should simulate Karliah pulling the chain to bring the bars down. Run inside and then bring up the console again and execute   and then   to teleport Karliah to you. Once you and Karliah are both inside execute   to progress the quest.
 
Move around a little until the cinematic begins.
  If the above fails, make sure you have exhausted all dialogue with Karliah and try again.
 
Quickly talk to Karliah before she can pull the chain a second time.
 
Have your followers wait outside.

Quest Stages