Series: Touhou - Until Kirisame Marisa Becomes a Strange Creature (Nodoguro (Phi-tan))

Artist: Nodoguro (Phi-tan)
Original title: 霧雨魔理沙が異形になるまで
Alternate title: 或ﾙ魔女ﾉ×罪 (A Certain Witch x Sin)

Prequel to the series Wonderful Shrine Maiden of Paradise vs Creature that used to be an Ordinary Magician. Currently ongoing in twitter/pixiv with 3 chapters posted.