Trinity Restored

Listen to Karliah
Upon meeting with Brynjolf and Karliah in the center of The Ragged Flagon - Cistern, a discussion ensues about executing Mercer for his betrayal to the Guild. However, Karliah warns the Dragonborn that, since Mercer was a Nightingale, he is very dangerous. Karliah wants to prepare the Dragonborn to slay Mercer and suggests meeting her at a standing stone just outside the southwest gate of Riften.

Meet Karliah at the Standing Stone
After proceeding to the standing stone, and discovering that it is the secret headquarters of the Nightingales, Karliah explains that the three will need all the help they can get to kill Mercer.

Follow Karliah
As the Dragonborn makes their way into Nightingale hall with Brynjolf and Karliah, they 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 arriving at the hall, Karliah requests that the Dragonborn go and don the new Nightingale armor to begin the ceremony.

Activate the Armor Stone
Once making it across the wooden bridge and into a room, there will be a stone off to the left with the Nightingale symbol on it. Upon activating it, a full set of Nightingale Armor is added to the Dragonborn's inventory.

Equip the Nightingale Armor
Once the armor is equipped, the ceremony can begin.

Follow Karliah
Karliah tells the Dragonborn of some tests that need to be completed in order to become a Nightingale, while Brynjolf displays negative feelings towards this. Karliah explains that the only way to defeat Mercer Frey is to have Nocturnal on their side. Karliah explains that in order to use the Nightingale abilities whenever possible, the test must be completed for a guardian of the Twilight Sepulcher serves both in life and death. Karliah asks if the Dragonborn is ready for the test and once conceding, the test can begin.


 * Note: Unlike other quests, the option of saying no does not appear at what should be a point of no return. At this stage of the quest, the only dialogue option there will be 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 three glyphs. All three characters take their place on one of the glyphs, and Karliah calls upon Nocturnal, who then appears. Karliah offers to have Brynjolf and the Dragonborn take the oath to serve Nocturnal in order to gain the strength to take down Mercer Frey. Nocturnal accepts the offer and the three characters become the new Nightingales.

Speak to Karliah
Now that the Dragonborn is 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. 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
Brynjolf then realizes that, because the Guild is leaderless, the Dragonborn will need to take over as leader of the Thieves Guild after Mercer has been dealt with. He then mentions that they should head to Irkngthand where Mercer is likely to be, stealing the Eyes of the Falmer.

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.
 * It is possible to take off the Nightingale armor upon making it to the room where Nocturnal comes. There, any armor can be worn without stopping progress in the quest.
 * If Serana is a follower, she may constantly interact with the pull chain, but the gate will not open.

Bugs
Die Wiederherstellung der Dreieinigkeit Trinità Ripristinata Возрождённая триада