That Time I Got Reincarnated as a Slime, Vol. 13 (light novel)  (English, Paperback, Fuse Kevin)

The Empire deploys a vast army over 900,000 soldiers strong to invade theForest of Jura. In response, Ramiris evacuates Tempest's inhabitants into theDungeon for safety. While Primal Demons Testarossa and Ultima duke it outwith the Empire on the front lines, the rest of the imperial army attempts toinvade Ramiris's labyrinthine shelter. Little do they know, however, that theonly thing they'll find there is a devastating bloodbath courtesy ofTempest's own mighty forces...