Encephalopathy Domination
By then, the remaining tens of millions of demons would show up and finish them up. That was, after breaking their will and crushing them thoroughly. But things didn't go as planned.
That attack alone had the potential to put half the entire Region in turmoil. Let alone this small hemisphere, he had to do something about it. Which was why he used his unique skill to silence everything.
As the Noble Demon King of Silence, something like this was of course not that much trouble. But if little Kappa kept firing them out like an artillery, then he would not hesitate to turn tail and run.
As a peak Regal Noble demon king, his demonic power reserves were capable of cloaking the whole region. Possible almost a hundred regions if it was not for the membrane separating every region from the next.