A "what if" alternate story that's likely a previous timeline that Homura went through on her way to the events of the Anime storyline. Very interesting, and surprisingly true to the characters in a way that makes sense and doesn't feel like "fanfiction" the way some other spin-off manga do.
Feels very grounded in the world building of Madoka Magica, and characters act really true to themselves. Worth a read if you don't mind basically seeing an "alternate route" to the events of the original storyline.