Dakara Watashi wa Mahou Shoujo wo Yameta

One day, Torika loses contact with her friend Watanabe-chan.

Following the last message she finds, she heads to the aquarium, only to find that the aquarium has turned into a labyrinth.
While searching for her friend and walking through the labyrinth, she encounters a monster.

The monster suddenly appears, and a strangely dressed woman fights it.

The woman calls herself a magical girl, and uses fire to repel the monster. She encounters a monster called "Kuiyose" and the "Magical Girl" who fights it.

It was the beginning of a new era that would repaint her everyday life.

[From DLsite]