That Time I Got Reincarnated as a Slime, (Light Novel) Vol. 14

Having easily thwarted the Eastern Empire's invasion, Rimuru wishes to end the conflict once and for all and therefore heads for the imperial capital to take out the source. Meanwhile, Yuuki has worked his way up inside the Empire with the intention to stage a coup d'état and usurp the throne. Rimuru, however, comes to find that the Empire's true power is on a whole other level from what he’s seen so far...