That Time I Got Reincarnated as a Slime, Volume 16 (manga) — Fuse

In the aftermath of Rimuru's ascension to Demon Lord, his friends and allies are taken aback by his newfound power, but even more shocked at the return of the dreaded Storm Dragon, Veldora. Meanwhile, the other Demon Lords are all too wary of Rimuru's evolution, which upsets their delicate balance of power and seems to be a harbinger of greater struggles to come...