Trinity Restored Trinity Restored is a quest available in . Walkthrough . 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 .