Warning, Minor Spoilers Ahead!
After completing The Pursuit, the next task is to take vengeance on Mercer for his betrayal, but Karliah understands that this will be no easy tasks and wishes to even up the odds. She asks you to meet her just outside of Riften.
Once you arrive, both Brynjolf and Karliah will be waiting for you. Karliah explains that this is the headquarters of the Nightingales, agents of Nocturnal, a title that Mercer also happens to hold. Karliah has brought you and Brynjolf here in order ask Nocturnal to become Nightingales.
Follow Karliah inside and do as she says; eventually you will be given a set of Nightingale armor. Continue to follow the events of this quest to learn that Mercer has stolen the Skeleton Key of Nocturnal which must be retrieved.
Note: the Skeleton Key of Nocturnal doubles as the only unbreakable lockpick in the entire game. If you wish to complete the Thieves Guild Questline, you must return the key to Nocturnal, but this can be done at any time.
Skyrim Sidequest - Trinity Restored,
Depending on how many subquests you have completed for the Guild, Brynjolf will indicate that you are to become the Guild Master, or he will say that when the guild is back on its feet (meaning you have not completed enough subquests) you will become Guild Master. This will complete this quest and begin Blindsighted.