The Lemon Demon, a surprise antagonist in the world of FNF, only had one song before, but now he comes around with a full mod dedicated to you battling him on all the following songs:
Defeat the Lemon Demon of FNF in a full week!
From the main menu you have to choose between the story mode and free play mode, and in either case, know that to win you need to do the same thing, which is to reach the end of the songs by playing your notes according to the charts.
How? Well, when you see arrow symbols float and match above BF's head, that is when you need to press the identical arrow keys to hit your notes. Miss doing it too many times in a row, and you lose, so be careful not to let it happen. Enjoy!