Also referred to as the Blank Century, The void century was hinted at during the Skypiea Arc; the dates were seen in Robin's flashback. The years of the void century rage from 700-800 in the Age of the Sea.
This period became known as the blank century because there is nothing left to acknowledge anything that happened, aside from the Poneglyphs which are written in a language that few know how to read.
However, it causes a problem with the known histories of the world as nothing ties up because of that missing period, although most people do not seem to be aware about this information. For example, Nefertari Cobra after being told by Nico Robin about this missing period was completely shocked by its existence and didn't know there were parts of the world history that didn't tie in.
The Great War
The most major event during the Void century seems to be the Great War.
The great city of Shandora collapesed during this war, the Shandorians defended the city against those "who seek the stone". The city collapesed but the Shandians, who are decends form the Shandorians, kept on defending the city for over 800 years
The real reason why the World Government banned research on the Phonegyphs, the Government lied to the world and uses the Ancient Weapons as a cover-up of their real reason. The Government is afried of the ideals that the kingdom holds, what threat of this ideals has yet to be revealled.
This worldwide kingdom was first mentioned in chapter 395 by Professor Clover who tried to tell the Gorousei the real reason why they were afraid of the Void Century. Before Clover can say the name of the kigndom, he was shot.
What little facts are known are that it held great power until the Alliance, who fought against the kingdom, managed to defeat them. However, the details of the events leading up to the defeat and how they were defeated remain unknown.
The Alliance are the ancient forces that became the World Government. During the Void Century they fought against the Ancient Kingdom. Who are the Alliance and what is the connection with the Ancient Kingdom remains unknown as of now.
The legacy of the War
Birth of the World Goverment
In 800 at the end of the Void Century, the World Government was born and took political control of the entire world, uniting all countries and forming the Counsel of Kings. For the World Government, the events of the void century were better left unknown as information linked to it is considered far too dangerous.
For the main article, see Poneglyph
Only 2 weapons are metioned but were never shown. The Ancient Weapons known in the story so far are Pluton and Poseidon. Each weapon was constructed during the void century and so far all mentioned weapons were capable of extreme mass destruction, such as the complete destruction of entire islands.