Nightingales

"Our trinity serves the Lady Nocturnal the Empress of Murk and the Daughter of Twilight. We believe her to be our patron, if not the patron of all thieves worldwide. We serve her without prayer, without charity and without celebration."

- Gallus Desidenius

Based in Nightingale Hall, The Nightingales are a secret inner circle within the Thieves Guild. The most recent incarnation of the faction was made up of three elite thieves: Mercer Frey, Karliah, and Gallus. The Nightingales exist to protect the shrines of Nocturnal throughout Skyrim.

As the Dragonborn progresses through the Thieves Guild, they are invited to join the Nightingales. In doing so, they swear to protect Nocturnal and her shrines, even after death. As a reward for the pledge of service, Nightingales are granted armor blessed by Nocturnal, and receive special powers, including increased luck in their exploits as thieves.

After the completion of Trinity Restored the Dragonborn becomes a fully-fledged Nightingale. They are then allowed to choose what manner of agent of Nocturnal they want to be (see Powers below).

History
According to "Nightingales: Fact or Fiction?", the first signs of the existence of a group dedicated to the service of the daedric prince Nocturnal - the Nightingales - surfaced over one-hundred years prior to the events of. A corpse was discovered wearing strange black armor, and bearing an insignia that resembled a nightbird cradling the moon in its wings. Since then, it has always been rumored that a secret inner-circle existed within the Thieves Guild which was responsible for protecting the shrines of Nocturnal. Little is known of the Nightingales' membership prior to the return of Alduin, but purportedly a Dunmer named Lorthus who was executed for murder, was suspected of being a Nightingale.

Powers
There are three known kinds of Agents, each possessing a power that can be used once a day. It is possible to change to another kind of Agent, but only once per day, and only at the Twilight Sepulcher.

Agent of Stealth (crescent moon)
An Agent of Stealth is an expert infiltrator. They use darkness to conceal themselves. They are granted the power of Shadowcloak of Nocturnal which automatically turns the Nightingale invisible when sneaking. While sneaking, the effect acts as any other form of invisibility, meaning that if the Agent interacts with an object or character, the effect will dissipate. However, the cloak can be re-engaged by leaving and entering sneak again (as long as the duration holds). Shadowcloaks last for two minutes.

Agent of Subterfuge (half moon)
An Agent of Subterfuge uses darkness to obscure others' thoughts and judgement. They are granted the power of Nightingale Subterfuge which forces targets in the area of effect to attack anybody on sight, allowing the Agent to take advantage of the chaos to gain the upper hand.

Agent of Strife (full moon)
An Agent of Strife uses darkness for their own benefit. They are granted the power of Nightingale Strife that creates a cord between them and their foes. This cord draws health from said foe while giving health to the Agent.

Trivia

 * While wearing the Nightingale Armor or Nightingale Hood, thieves may pay homage to the wearer in the form of 500 or other gifts. They flee afterward so long as they are not attacked.
 * When you choose the Nightingale Power, If you choose the Agent of Stealth and then wait for 24 hours then choose another power, whenever you sneak you will be invisible and have another Nightingale Power.
 * During Sovngarde, Tsun asks the Dragonborn why they should be allowed to enter the Hall of Valor. The Dragonborn can reply that they are a Nightingale of Nocturnal.
 * Gallus's Encoded Journal has a unique cover that is adorned with the Nightingales' emblem. (Pictured)

Bugs

 * The Shadow Warrior perk (from the Sneak tree) conflicts with Shadowcloak of Nocturnal. This occurs because of the way the Shadow Warrior perk works. (Shadow Warrior causes you to go invisible briefly, causing enemies to pick a different target.) With Shadowcloak of Nocturnal active, crouching when enemies see you will cause both Shadowcloak of Nocturnal and Shadow Warrior to make you go invisible, but after about half of a second, Shadow Warrior removes the invisibility. (This does not always work & has not been confirmed)
 * To fix this, remove the Shadow Warrior perk using the console command "player.removeperk 00058214". Then use the Shadowcloak of Nocturnal and it will behave normally. Once the skill is finished, re-add the perk with "player.addperk 00058214". By doing this once, it appears that this bug is resolved. Shadowcloak of Nocturnal will override Shadow Warrior when it is active from this point forward and players will not have to remove/re-add in future instances of using the skill.
 * Another way to fix this, just Sneak and interact with anything, you when you get up and sneak again you will not become invisible.