This quest starts anytime after you become a Nightingale. A courier delivers a letter from Brynjolf telling you to meet him at The Ratway. There he tells you about a legendary ring called, "Ring of the Nightingale's." He tells you he found a map and letter while going though Mercer Frey home. Then hands you the letter and map (you get a map marker.)
There you start a quest called: "Secret's of the Nightingale Order."
When you get there its a old Dwermor Ruins.
You well face many Falmer, and a group calling them selves "The Nightingale Order."
Mercer Frey (Let the storyline tell you why.)
Ring of the Nightingale's stats: