Abnormal Moon

This is a crossover anime/manga series with an infinite flow theme, starting with Type-Moon and later involving Senki Shurato, Jujutsu Kaisen, Overlord, and others.
The earth died, the sky was obscured, and a filthy curse spread across the globe. Monsters roared in the wild, evil spirits lurked in the shadows, cursed spirits hid in the darkness, demons were arrogant in hell, and corrupted Servants patrolled their territories... Humanity suffered a great defeat, and the world's population dwindled to less than one-hundredth. The survivors could only eke out a living by hiding in base cities.
Fuyuki City is one of the four remaining base cities in Japan.
Having transmigrated here, Roy already felt something was very wrong, but when he saw Kasumigaoka Utaha, Hayasaka Ai, Nakano Miku and others who had also transmigrated here, he realized that it wasn't just the Type-Moon world that was abnormal; the entire crossover anime world was acting strangely.