An idea based on the game background of FSN.
Semi-empty.
Although the story takes place in the future, it does not describe future technology.
(That's not the point)
Some of them are purely based on imagination, so please don't delve into them.
BUG must be flying OTL (the possibility of a single person offering a demon is 0... It is the absolute biggest BUG, but if it is admitted, there will be no such story, so please close your eyes and let me give a far-fetched reason, and treat it as possible...)
PS: Be sure not to skip the prologue, there are some necessary instructions that need to be conveyed in the prologue.