Nocturnal is the Daedric Prince of Night and Darkness and is one of the most recognizable of the Daedric Princes thanks to her role as the patron of the Thieves Guild. All thieves' luck is drawn from her. The leaders of the Thieves Guild, the enigmatic Nightingales, are her direct servants in Tamriel and live to take care of her shrine, even beyond the grave when she claims their souls after death. As with all Daedric Princes, Nocturnal has several key artifacts attributed to her, such as the Skeleton Key which appears in Skyrim. As the patron of thieves, her sacred objects have a tendency to be stolen, as is the case with one of her most famous artifacts that should show up again in The Elder Scrolls 6.
The leader of the Thieves Guild in Cyrodiil, unlike in Skyrim, was not a Nightingale; instead, the guild is led by the Gray Fox, a figure who's existed throughout history for hundreds of years and who is famous for having stolen the Gray Cowl of Nocturnal from the Daedric Prince. The Cowl is one of the sacred artifacts of Nocturnal and was said to make the wearer invisible. While the Cowl doesn't quite do that, its effects are functionally far greater and have been coveted by thieves for centuries, and the Cowl would be a great boon in The Elder Scrolls 6.
The History of The Gray Cowl of Nocturnal in Cyrodiil
The Cowl was actually stolen by master thief Emer Dareloth, prompting Nocturnal to place a curse on the Cowl to strike the wearer of the Cowl from history, meaning they become a stranger to all who know them, even if they were to renounce it entirely or pass it on to someone else. This became a problem for owners of the Cowl and, by extension, the leader of the Thieves Guild, who would always be struck from history once they put it on.
Forced to exclusively assume the identity of the Gray Fox, the curse limited those who used it in several fundamental ways, the most notable being that their identity becomes completely limited to that of the Gray Fox, a notorious criminal, and cannot assume a civilian identity. This also means that they become a stranger to everyone who knows them. The events of the Thieves Guild storyline remove the curse, keeping the Cowl in the hands of mortals.
The Gray Fox's Potential in The Elder Scrolls 6
The Cowl now gives the wearer complete anonymity; any crimes committed while wearing the Cowl are attributed to the Gray Fox rather than the individual, meaning that any bounties incurred, or any bad deeds committed by the character, are erased once the Cowl is removed. As the player only obtains the Cowl after the removal of the curse, they get all the benefits and none of the drawbacks of becoming the Gray Fox.
The Cowl of Nocturnal is the most useful of all the Daedric artifacts, even more so than Nocturnal's other item, the Skeleton Key, which becomes redundant once the player character reaches a certain level in lockpicking, and needs to be returned to Nocturnal to complete the Thieves Guild quest line in Skyrim. With the Cowl of Nocturnal traditionally in the hands of mortals, there's no reason to return it to Nocturnal and there's no reason for it not to be in play during the events of The Elder Scrolls 6.
An intersection of the Gray Fox and the Nightingale storylines could be an interesting conflict for the Thieves Guild in The Elder Scrolls 6 and bring back a figure many players and NPCs wanted to see in Skyrim. The Gray Fox is a legend in The Elder Scrolls lore, and a comeback in Hammerfell would be welcomed by many players, especially if it comes with outwitting Nocturnal once again.
The Elder Scrolls 6 is in development.