What I do remember is sparse and weird. There was a lot of death of humans by monsters; monsters that turned out to be evolved humans or something weird? The manga is completely translated, and at the end it was revealed the world was actually post modern times and not some fantastical alternate world. Human history had been altered by the villain, who ended up in a museum glass box as just a brain that if you pressed a button would be tortured (I think). It's bothering me like crazy that I can't remember what manga this was! Thank you in advance.

Shin Sekai yori?

YES! Thank you so much!