I'm not sure specifically as I haven't read it. But it might be about a woman who is married to a character symbolized as the 'Prince of Darkness', perhaps exploring themes like love in an unexpected or dark context, or the struggle between good and evil within a relationship.
It depends on various factors. Some fanfiction works gain popularity within specific fan communities. 'Harry Potter' has a huge fan base, but a 'Prince of Darkness' - themed fanfiction might be popular among those who like darker, more alternative takes on the story. However, it may not be as mainstream as some other more canon - compliant fanfics.
In Harry Potter fanfiction, 'The Prince of Darkness' could refer to a number of things. It might be a new character created by the fanfiction writer, perhaps a dark wizard or a powerful, malevolent being. It could also be a title given to an existing character in a new, dark context, like Voldemort being given this title to emphasize his extreme evilness.
It could be a new villain that the fanfiction author has introduced. Maybe a long - lost relative of Voldemort who has emerged to continue his dark legacy.
I'm not entirely sure if there is a specific 'prince of darkness naruto x hancock fanfic' directly. But given the popularity of both Naruto and Hancock characters, many fans like to create crossover stories. It's possible that on specialized Naruto or One Piece (where Hancock is from) fanfic communities, or even general anime fanfic forums, you might stumble upon such a story. You could also try reaching out to fellow fans on social media platforms and ask if they know of any.