That Time I Got Reincarnated as a Slime Manga, Vol. 17

This volume puts the spotlight on how the humans have completely underestimated Tempest. Each of Rimuru's evolved followers carried out the simultaneous counterattack. One the best moments of the series so far, laced with heavier and darker themes. Two additional elements have been introduced - a powerful and sinister human mage and actual demonic entities. So far, we've only heard the term “demon lord”, but actual demons appeared. And based on the dialogue, we can expect something very significant to come from these summoned demons.