Trinity Restored

Listen to Karliah

 * Journal Entry: Before intercepting Mercer Frey, Karliah wishes to speak to both Brynjolf and myself.

Meet up with Brynjolf and Karliah in the center of the Ragged Flagon - Cistern. They talk about killing Mercer for his betrayal to the Guild but Karliah warns you that he is a Nightingale and is highly trained. Karliah wants to prepare you before going out to kill Mercer and asks you to meet her at a standing stone just outside the Southwest gate of Riften.

Meet Karliah at the Standing Stone

 * Journal Entry: Karliah has indicated Brynjolf and I are to meet her at an old standing stone just outside the Southwest Gate of Riften. Her motivations for doing so are mysterious, but I suspect it has something to do with the Nightingales.

After making your way to the standing stone, you discover that it is the secret headquarters of the Nightingales. Karliah explains that we will need all the help we can get in order to kill Mercer Frey.

Follow Karliah
You make your way into Nightingale hall with Brynjolf and Karliah who begin discussing the Nightingales and the myth behind them. Brynjolf believes that the Nightingales are a religious group, but Karliah explains that it is all about business. Once you are at the hall, Karliah requests that you go and don your new Nightingale armor to begin the ceremony.

Activate the Armor Stone

 * Journal Entry: Karliah has informed Brynjolf and I that we're to join the ranks of the Nightingales and assist in reforming the trinity that was disrupted over twenty-five years ago when Mercer Frey murdered Gallus.

You make your way across the wooden bridge and into a room on the left where you find a stone with the Nightingale symbol on it. Upon activating it, a full set of Nightingale Armor is added to your inventory.

Equip the Nightingale Armor
Upon equipping the armor, the ceremony can begin.
 * Nightingale Armor

Follow Karliah
Karliah tells us of some tests that we must complete in order to become a Nightingale, Brynjolf is not pleased about this. Karliah explains that the only way to defeat Mercer Frey is to have Nocturnal on our side. Karliah explains that in order to use our Nightingale abilities whenever we wish, we must become a guardian of the Twilight Sepulcher in both life and death. Karliah asks if you are ready for the test and once you agree, the test can begin.


 * Note: Unlike other quests, you are not given the option of saying no at what should be a point of no return. At this stage of the quest, the only dialogue option you'll have is to either say "yes" or drop the conversation. However, this effectively stalls the entire quest line.

Stand on the Vacant Floor Glyph
Karliah opens a gate into a room with 3 glyphs. You each take your place on one of the glyphs and Karliah calls upon Nocturnal who appears before you. Karliah offers Brynjolf and yourself to take the oath to serve Nocturnal in order to gain the strength to take down Mercer Frey. Nocturnal accepts the offer and the three of you become the new Nightingales.

Speak to Karliah
Now that you are a Nightingale, Karliah reveals the true crime of Mercer. Mercer has stolen a Skeleton Key from the Twilight Sepulcher which not only unlocks physical locks but also the ability to tap into the potential that everyone possesses. This is the reason that the Thieves' Guild is down on its luck and if the key is not returned then its luck would go down to the point of non-existence. Returning the key would mean that Nocturnal would align itself with the Guild once again and luck would be restored.

Speak to Brynjolf

 * Journal Entry: I've undertaken the Oath and I'm now a Nightingale, guardian of the Twilight Sepulcher and a dedicated sentinel of the goddess, Nocturnal.

Brynjolf informs you that because the Guild is leaderless, you will take over as leader of the Thieves' Guild after Mercer has been dealt with. He then tells you to make your way to Irkngthand where Mercer is likely to be, stealing the Eyes of the Falmer.

Bugs

 * Karliah may not be waiting for you at the stone at the beginning of the quest. This may happen if you come across the nightingale stone before this quest.
 * Console command at the quest marker "player.placeatme 0001B07F."
 * Both Karliah and Brynjolf may not be waiting for you at the stone at the beginning of the quest. This may happen regardless of whether you found the Nightingale stone before. Even when you approach the stone and see both characters standing there, but do not go near enough to trigger the conversation but come back later, they might be gone. Brynjolf can then be found in the Ragged Flagon cistern while Karliah is in the winterhold's inn basement. The quest marker is fixed on the Nightingale stone and completing the quest is impossible. It is recommended that you immediately do enter the Nightingale Hall with them when you see the characters are there to avoid them disappearing later on.
 * There is a bug that occurs when Karliah is supposed to open the gate. Instead of opening the gate, Karliah will walk back to Riften. She is considered 'Busy', and so is 'Brynjolf' when spoken to. Brynjolf may be able to be fixed if he is pushed into the room with the runes in it, or PC players can use the "resetAI" command. PC players are also able to type in "setstage TG08A 57", which will advance the quest to the next stage, but will skip the Nocturnal cut-scene and may not open the chamber for later. Another fix is to stand closer to the gate while Karliah is speaking and asks you to accept the oath - standing on the steps and waiting for her to come to you is the simplest way to prevent her from wandering off. This will cause Karliah to open the gate rather than walk back to Riften.
 * Karliah can be fixed when she stops to have the final chat, in the chamber with the 8 fires. Stand with your back to the barred ritual chamber and your face pointing to the stones where you get your armor. She will start walking in the good direction, towards the barred door and you MUST save the game at this point. When she reaches the barred door, she will use the chain one time too many and the bars will drop and then raise again. When that happens, simply bump into her so that she opens the door but doesn't close it again. If all is well, she will resume her walk across the ritual room. Note: This requires a bit of precise timing and remember to block her path to the chain until she is pathed towards her destination.
 * An easier way to get around Karliah pulling the chain too many times (instead of trying to 'bump into her' at precisely the right time), is to just wait until she has pulled the chain once, and then quickly use Unrelenting Force to blast her into the next room. Once she picks herself up, the rest of the quest will continue normally.
 * You can also keep getting Karliah to ask you if you are ready to take the oath, but instead of answering, back out of the conversation and take a few steps backwards. Karliah will take a few steps towards you and ask you again if you are ready. Repeat this until she is just on the bottom of the stairs in the hallway leading to the barred door. Once you do this answer the question and she will continue to walk to the chain and open the gate.
 * Brynjolf can also appear in the cave, but turns around and heads back to Riften as soon as Karliah asks to pick up the armor. Karliah has a conversation with Brynjolf even though he's not in the Fire Chamber. Brynjolf appears briefly to accept oath but starts walking back to Riften again and not towards Ceremonial Chamber. You can force Brynjolf into Ceremonial Chamber by pushing into him while running. Force him up to the center platform next to Karliah and use Unrelenting Force to knock Karliah into water. Keep Brynjolf in the room by blocking him until Karliah returns to her spot. Once Karliah walks back to her spot, Brynjolf walks to his platform. Perhaps an easier way to fix the glitch is to push Brynjolf to his own platform, then use the ice form shout to freeze him in place, then go to your own platform and the cutscene will start.
 * There's a bug or glitch that there is will be an Imperial Courier appear inside and start running to the exit.
 * may fall through the floor or disappear while walking to the armor stones. Scripted events continue as if she is still present.
 * A simple fix to most bugs is to reload from a previous save point until the bugs stop occurring.
 * When you reach the room with the armor stones and Karliah has had her discussion with Brynjolf, instead of opening the gate she will just stand there repeating "It appears you are ready to take the oath."

Trivia
If the Dragonborn has acquired the Ice Form Shout, after completing the ceremony they can use it to freeze Brynjolf and Karliah and pickpocket them for items with no repercussions. After unfreezing and traveling to Irkngthand the quests will continue as though nothing had happened.