91 Days: After the End
Two years before the official end of the Prohibition Law, a budding writer stumbles upon a mysterious apartment while searching for a place to stay. There she discovers a long forgotten letter that sends her spiraling into a story of an old age war of revenge, deception, betrayal, and atonement...
Disclaimer: I own nothing. Credit goes to its respective owners. Also note, this is my own take on the open-ended scene in Day 12 of the anime and is in no way canon.