Skyrim:Nightingales

right|The crest of the Nightingale Trinity
The  , usually called simply the Nightingales, are a trio of highly skilled thieves dedicated to the service of Nocturnal. Members are typically chosen from the higher echelons of the Thieves Guild. Although there is no formal association between the two factions and most guild members are unaware of this arrangement, the Trinity is usually very influential on how the guild operates (Nightingales even claim to be the source of the guild's phrase "walk with the shadows").

Members of the order take the Oath: the Skeleton Key must remain at the Twilight Sepulcher, the Temple of Nocturnal, which must be protected from all threats, even in death, until Nocturnal accepts the spirit of a Nightingale into the Evergloam, her plane of Oblivion. In return, Nightingales use the abilities granted to them for whatever they see fit to do in life. Contrary to popularized misconceptions, the Oath is considered a business transaction rather than a religious matter. While most members certainly take the Oath very seriously, they are not worshipping Nocturnal as a deity, but respecting her as the patron of thieves. After their death they continue to serve Nocturnal as Nightingale Sentinels.

The Nightingales make their home in Nightingale Hall. The well-being of the Thieves Guild relies heavily on Nocturnal's content or displeasure with the current Nightingales according to Karliah. Even though this is the case, the Nightingales are a faction sworn to secrecy and their existence has purposely been made to seem like a legend by current and past members of the Nightingales. Although very few Guild members know of the Nightingales, many believe in the unquantifiable 'Luck' the Guild possesses.

Prior to playing the Thieves Guild questline the Nightingale trinity consisted of Gallus (a former Guild Master), Mercer Frey the current Guild Master, and a mysterious woman by the name of Karliah. Once engaged in the Main questline, you and Brynjolf take an Oath to serve Nocturnal and use the new advantage against Mercer Frey. Upon returning The Skeleton Key to the Sepulcher you are given a chance to choose one of three Agent abilities granted to those who serve Nocturnal. 

Agent of Subterfuge: The Half-Moon represents this power, Nightingale Subterfuge, which allows you to utilize shadow to cloud the judgment of others. It may be used once a day for 30 seconds in order to make people and creatures on the spell's area of effect attack anyone nearby.

Agent of Stealth: The Crescent Moon represents this power, the Shadowcloak of Nocturnal. Once a day for 120 seconds, entering Sneak Mode grants you Invisibility. Just as with invisibility potions or the Invisibility spell, the effect is canceled when you  attack, cast a spell, or activate anything in the environment (opening doors, operating switches, picking up items, talking, pickpocketing); however, the Shadowcloak invisibility can be renewed repeatedly during the 120 second duration simply by sneaking again.

Agent of Strife: The Full Moon represents this power, Nightingale Strife, which allows you to instantly absorb 100 Health points from a target once per day.

See Also
For more information, see The Nightingales, a two volume series of books about the Nightingales, and the lore page.