Great Kingdom

The "Ancient Kingdom", whose name remains a mystery, was a kingdom said to be enormous, prosperous, and powerful. It was likely an enemy of the 20 Kingdoms, (what is now the World Government) as implied by Professor Clover, and was seemingly utterly destroyed by them over 800 years ago at the end of the Void Century. In fact, it seems their defeat allowed the Ancient Alliance to become truly the World Government. Furthermore, it is the real reason the World Government has banned any Poneglyph related research as the mere "existence and idea of this kingdom presents a threat to (the) World Government".

The "Ancient Kingdom" was first mentioned by Professor Clover amid the impending destruction of Ohara as he explained to the Gorousei that he knew why they are afraid of the history covering the Void Century, as it would surely speak of this kingdom said to hold mighty power and ideals which, if discovered, posed a threat to the World Government.

Knowing it might lose the war and that all other information would eventually be destroyed by their enemies, the kingdom arranged for their writings to be carved onto indestructible stones, the Poneglyphs, as a means of passing messages on to future generations. Sure enough, the World Government later banned any research of the Poneglyphs and methodically erased the information on the "Ancient Kingdom" from all historical records and texts. The Poneglyphs are now the only remaining source of information on the "Ancient Kingdom" or the Void Century.

As the Poneglyphs are considered indestructible, the fact that this "Ancient Kingdom" was able to not only create, but also shape and carve them, gives a small insight into the potential technology and abilities they had. The Poneglyphs, thus far, has uncovered weapons such as Pluton and Poseidon. It may be possible that the people of this kingdom were somehow involved in the design and/or creation of these enormously powerful weapons. Again, this suggests that, as a people, they may have been significantly advanced in technology.

It's unknown why knowledge of this kingdom is a threat to the World Government. Professor Clover seemed certain that the history of the kingdom held the key to the answer to that and other pertinent questions. However, before Clover could say the kingdom's name, he was shot. Further information has yet to be revealed.