It was likely an enemy of the Twenty Kingdoms (which is now the World Government), as implied by Professor Clover, and was seemingly utterly destroyed by them over eight hundred years ago at the end of the Void Century. In fact, it seems their defeat allowed the Ancient Alliance to truly become the World Government.
Furthermore, it is the real reason the World Government has banned any Poneglyph deciphering and research as the mere "existence and idea of this kingdom presents a threat to the World Government".
The "Great Kingdom" was first mentioned by Professor Clover amid the impending destruction of Ohara as he explained to the Five Elders that he knew why they are afraid of the events during 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 "Great Kingdom" from all historical records and texts. The Poneglyphs are now the only remaining source of information on the "Great Kingdom" or the Void Century.
The Poneglyphs so far have revealed information on the three Ancient Weapons, all of which are capable of mass destruction on a global scale. 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. However, the intended purpose of these weapons, as well as why the kingdom had built them has yet to be discovered.
It is 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.
- ↑ 1.0 1.1 One Piece Yellow: Grand Elements (p. 276), The kingdom is referred to as the Great Kingdom.
- ↑ 2.0 2.1 2.2 One Piece Manga and Anime — Vol. 41 Chapter 395 and Episode 277, Clover speaks about the Great Kingdom to the World Government.