Quick answer
What is known about the Ancient Kingdom?
The answer
The Ancient Kingdom was an enormous prosperous country defeated 800 years ago whose existence threatens the World Government.
Verified through chapter 395Updated
What the manga shows
4 factsProfessor Clover states that documents and Poneglyphs verify the existence of this enormous kingdom from the Void Century.
Clover notes the kingdom prided itself on great power yet all information about it was erased from history.
The kingdom was defeated by enemies who became the World Government after the Void Century ended 800 years ago.
Clover claims the ancient kingdom's existence threatens the World Government.
Change log
1 chapter- Chapter 395Added 4 facts; updated the answer