Although not written by Isayama Hajime this is a story that is considered part of the Attack on Titan canon (according to wiki) and is a spin off that tells the story of Levi and Erwin’s past and how Levi joined the Survey corps.
You may feel your favorite manga should be gathered together into distinct categories for your own reference and, now, you can do this with a LIST. After you've created your list or lists, you can proudly recommend them to other manga fans to showcase and share your taste in manga.
Phantom in the Twilight