That Time I Got Reincarnated as a Slime (light novel) - Volume 5

Despite Rimuru's absence, Tempest was as peaceful as always. Well, as peaceful as a monster town called "Tempest" could get, anyway. But that was only until they learned that a bunch of heavily armed humans were coming their way. To make matters worse, the Beast Kingdom of Eurazania has declared war on Milim, and Rimuru is nowhere to be found...!