Youkai Gurentai summary:

One day, at the base of Mt. Fuji, appeared a "castle" pouring out Youkai. To curb the labour shortage caused by a declining population, the government decided to integrate the Youkai into society; however, this led to rising conflicts between Humans and Youkai.

In response, the JSDF, with aims to expand their influence in public security, secretly forms a Youkai penal tactical response force, "Tachikawa Task Force", to execute "tactical solutions" for the various issues born from this new society.