The Monster of Yuuei (смешанная)

Monsters were things that waited in the shadows, attacking humans and leaving chaos in their wake. They were called Nomu: mindless, terrible creatures from the east that instilled fear in the Kingdom of Yuuei.
Except there was one monster that always showed up in a whirlwind of smoke and black tendrils, ruthlessly tearing apart any Nomu that crossed into its territory, before disappearing into the night. The Monster of Yuuei, as they called it, never seemed to attack humans, but it was frightening all the same.
And it doesn't take long before a secretive forest witch, the captain of the Royal Guard, and the crown prince realize that there's way more to this monster than it seems.
(Or: A story where Ochako stumbles upon a gravely injured monster and discovers the lonely boy underneath, and Katsuki tries to find ways to stop the invading monsters while keeping his idiot boyfrie--err, Prince safe.)