Nightingales

The Nightingale are a splinter section of the Thieves Guild, the group was made up of three elite thieves: Mercer Frey, Karliah, and Gallus. The purpose of the Nightingales is to protect Nocturnal and the shrines to her throughout Skyrim.

As one progresses through the Thieves Guild, they are inducted into the Nightingale order. When one does this, they swear to protect Nocturnal and her shrines, even after death. In return for their oath, they are granted various powers of darkness and stealth.

After the completion of the Thieves Guild questline the player becomes a fully-fledged Nightingale. They are able to become an agent of the Nocturnal. There are three kinds of agents that one can become, each with a greater power that can be used once per day:

Agent of Stealth - An agent of stealth is the best infiltrator around. They use darkness to conceal themselves. They are granted a greater power called "Shadowcloak Of Nocturnal" which allows them to turn invisible.

Agent of Subterfuge - An agent of subterfuge uses darkness to obscure others thoughts and judgement. They are granted a greater power which forces their enemies to fight amongst themselves, making any fight much easier.

Agent of Strife - An agent of strife uses darkness for their own benefit. They are granted a greater power that creates a cord between them and one of their foes. This cord draws health from said foe while giving health to the agent.

One can choose which kind of agent to become, but the descision is not permanent. You can change which kind of agent you are once per day; however, you can only be one type of agent at one time.

Note: While Mercer Frey, Karliah, and Gallius were Nightingales, it can be inferred which Agent ability they took. Karliah is the Agent of Stealth, when she uses her Shadowcloak of the Nocturnal when you first meet her with Mercer and she escapes. ​Mercer Frey is an Agent of Subterfuge, which is shown when you confront him and he uses his Subterfuge ability to cause Brynjolf to attack Karliah. It is assumed Gallius was the Agent of Strife if this pattern continues.

When you first meet Karliah she uses a potion to disappear. The Nightingales have lost their powers because the skeleton key is the link that lets Nocturnal give the Nightingales their powers. Mercer, however, has all three of the powers and can use them at the same time. During the night he uses cloak to evade you, if you engage him head on right away he uses strife and he uses subterfuge to get you alone in the fight.

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.
 * (Untested) In the aforementioned bug, if players are playing the PC version of the game and encounter it, they can attempt to 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.