Nocturnal

Nocturnal is the Daedra lord whose sphere is the night and darkness. She is also known as the Night Mistress and "Lady Luck". The Cyrodiilic Thieves Guild leader, The Gray Fox, has the Gray Cowl of Nocturnal, which truly represents the sphere's darkness quality; it permanently removes the wearer's specific identity from all history. Regardless, the Cyrodiil Thieves Guild venerates Nocturnal by ending talks with the saying "Shadow hide you." The summoning date of Nocturnal is 3rd of Hearthfire.

Description
"Night among strangers. Secrets in the dark. Nocturnal is here."

- Nocturnal

Physical descriptions of Nocturnal are difficult to come by, as she is usually described as being hidden by darkness and shadow. Depictions of the Daedric Prince usually take the form of a nondescript human woman, draped in a hooded cloak that hides most of her features and body. She is depicted with both arms outstretched, and a pair of ravens (or similar bird) perched on each wrist.

Nocturnal claims to be an aspect of the void of Oblivion itself, and takes such titles as Mother of Night or Night Mistress. Her very nature is such that humans cannot understand her, as she is the embodiment of the unfathomable and enigmatic. Worshipers of Nocturnal consist primarily of those who operate in darkness and night, such as thieves and spies. Note, however, that Nocturnal's deception is not usually meant to be harmful or violent in the manner of Boethiah. Rather, the unknowable nature of Nocturnal is the core of his being, and serves no real ulterior purposes beyond being mysterious. Thus, while thieves may consider Nocturnal their patron, assassins will typically lean towards a more violent deity.

Worship
Although Nocturnal has arguably one of the largest followings of all the Daedric Princes, she has no form of organized clergy. There are a few who call themselves priests of Nocturnal, though the Night Mistress does not herself seem to show them any particular favor. Indeed, Nocturnal seems to have no real interest in worship. A temples of Nocturnal does exist called the Twilight Sepulcher, located in Skyrim, though this was built around the portal known as the Ebonmere, the portal to Nocturnal's plane of Oblivion known as Evergloam. Supposedly, Nocturnal is the source of a thief's luck, which flows from the Ebonmere and plays a heavy role in the lives of any who use the shadows to do their work, though none know what price is paid in return.

Nightingales
Though Nocturnal has no truly organized clergy, she has three mortal servants upon Nirn who are known as the Nightingales. The Nightingales serve Nocturnal directly, granted the power over shadow, subterfuge and strife in exchange for their service. Once a Nightingale enters into their contract with Nocturnal, they are bound for life and beyond to serve the Night Mistress in guarding the Twilight Sepulcher and in Nocturnal's realm of Evergloom.

Artifacts
"To steal from a god is most unwise."

- Nocturnal

Cowl of Nocturnal
The most famous feature of Nocturnal is the enchanted cowl she created. This headpiece possessed the ability to completely hide its wearer's identity from mortal knowledge, including magical forms of scrying and identification. The cowl's power was so great that people would be incapable of even remembering the name of whomever possessed the cowl, even while the cowl was not being worn. No one would be able to retain any long term memory of interacting with the cowl's wearer, and even removing the cowl in plain sight of others would not break its spell.

The cowl was reportedly stolen by the head of the Thieves' Guild (Emer Dareloth) during the last Third Era, promptly removing any memory of the person in question from history. Henceforth, according to legend, the bearer of the cowl was known only as "The Gray Fox", though it was not commonly known that the cowl was responsible for his infamy.

The cowl was also told stolen in one amusing song called "Song of Hrormir." Nocturnal was tricked by Hrormir in order to save his old companion Darfang. She traded Darfang's soul for the service (by keeping his soul) of Hrormir, so that he would assist her followers dividing the kingdom of Aelfendor, but in the end the kingdom was left intact while Nocturnal's followers were divided. And the Gray Cowl of Nocturnal was also taken by Hrormir. To get the cowl back, Nocturnal was forced to release Hrormir's soul and left with nothing to gain.

Skeleton Key
The Skeleton Key was reportedly owned by a mortal for the first time by an obscured person named Arrovan. Many years later, the Eternal Champion during the Imperial Simulacrum, found a map in a dungeon in Valenwood that pointed to the location of the Skeleton Key somewhere in Summerset Isle. Using the map, the Eternal Champion found and claimed the key. Then sometime in 3E 397, the Hero of the Azra's Crossing found the Skeleton Key in one of the chests in the Citadel of Broken Wing. Indeed the artifact travels between heroes and locations, as the legend says, it would never be the property of one thief for too long.

In Skyrim the Skeleton Key was given a new level of importance and power. In the Thieves Guild storyline, the Dragonborn discovers that not only can the Key unlock physical barriers, but also metaphysical ones. As demonstrates by Mercer Frey in the Blindsighted quest, the Skeleton Key also unlocks human potential.

Frey's theft of the Skeleton Key from the Twilight Sepulcher's Inner Sanctum in the Fourth Era is said the be the reason for the hard times that have fallen on the Guild in Skyrim, since the Ebonmere, the gate to Nocturnal's plane of Oblivion, was kept open by the Skeleton Key, allowing luck to grace the actions of thieves throughout Nirn. This may possibly make the Skeleton Key the most powerful Daedric artifact in existence given the sheer number of uses that it seems to have.