I'm not sure specifically as I haven't read it. It could be about various things. Maybe it has a science - fiction theme related to ultraviolet light, like some sort of adventure in a world where ultraviolet phenomena play a major role.
Well, without having read this particular 'naruto descendant of ultraviolet fanfic', it's hard to say precisely. It could be that the author has created a new character who is a descendant in the Naruto world and this character has some unique abilities or a backstory related to ultraviolet. Maybe it's a story about how this descendant uses their special connection to ultraviolet to face challenges in the ninja world, like fighting against powerful enemies or uncovering hidden secrets.
Since we lack the details of the 'Mona Darling full story', it's difficult to define who Mona Darling is precisely. She might be a protagonist in a love story, facing challenges in her relationship. Or she could be a strong - willed character in a thriller, fighting against some sort of danger. There are countless possibilities depending on the nature of the story.
Some fanfics may put them in a different universe altogether. Maybe a post - apocalyptic world where Flash's speed and Ultraviolet's abilities are crucial for survival. They might have to team up against new and powerful enemies that are unique to this new setting.