The R-18 arm of Syosetu. Generally denotes dark, decadent, sensual, erotic content. Harlequin also had this 'nocturne' division, I believe.
Nocturne is night, you must go into nocturnal mode into nocturne activity And it has friend with alphabet, like Nocturne in D minor, E flat or C sharp minor
https://www.reddit.com/r/copypasta/comments/got6q4/umbreon_vaporeon/ Aye, although my personal favourite is Devil Survivor 1