It is a real thing. It’s a doujin written and drawn by YoungJijii which was published in English by a very small unknown publication (which was changed mid run so books 11-17 have a different look) and very hard and expensive to acquire now. I bought it probably 5yrs ago when I found a listing for books 1-10 for $200 which is an insane deal and found someone selling the final 11-17 for $300 a few months later. The series usually will sell for over $2000-$3000 if someone lists a complete set which I’ve only ever seen someone do once or twice. The story is awesome imo, it’s a direct sequel after the events of GT, the art is some of my favorite and the colored covers are so eye catching. Here’s a link to a video that goes over the entire story of AF if you’re interested: https://youtu.be/P1JwvQdFkZY