That Time I Got Reincarnated as a Slime: Trinity in Tempest (manga), Volume 2

As they sharpen their battle skills and explore the power of magic, Phos, Stella, and Nemu continue to enjoy life in Tempest. But a shadow has fallen across the city, as humans bring their strange and threatening ways into the monster utopia. Trouble is also brewing between the monster kingdoms. Will fast friends soon find themselves to be mortal enemies?