Mahou Shoujo Yamada: Aru Manga Henshuusha no Kiroku

Satou is a manga editor who suffers from "Magical Girl Phobia" triggered by dreams featuring a mysterious magical girl. However, one day, he comes across a manga featuring illustrations strikingly similar to the magical girl from his dreams. Determined to uncover the cause of his phobia, he contacts the manga's author. But this marks the beginning of a descent into a dark, profound despair.

(Source: Coamix, translated)