Void Century

The Void Century, also referred to as the Blank Century, was hinted at during the Skypiea Arc; the dates were seen in Robin's flashback. The years of the void century rage from 600 AOS-700 AOS in the Age of the Sea. It is a century long gap in recorded and archaeological history, the study of which is forbidden by the World Government.

About
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 collapsed during this war, the Shandorians defended the city against those "who seek the stone". The city collapsed but the Shandians, who are descendants form the Shandorians, kept on defending the city for over 800 years

Moon
In Enel's cover story, there is an ancient city on the moon with other bunch of identical Spaceys. There is a painting on the wall that shows a Skypiean with Spacys following him on the left while two Jayans (presumably the Shandorians, now Shandians) are building a new Spacey. This suggest that the Skypieans and Shandorians may had worked together during the void century. How did the city get on the moon and why both races worked together remains a mystery.

However the like the Shandians, the Shadorians have wings.

Interestingly, nearly at the end of the Skypea arc, after Montblanc Cricket confirmed the existence of the city of Gold he said that the Sky Island has an "old and boring history".

Ancient Kingdom
The real reason why the World Government banned research on the Poneglyphs, the Government lied to the world and uses the Ancient Weapons as a cover-up of their real reason. The Government is afraid of the ideals that the kingdom holds, what threat of this ideals has yet to be revealed.

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 kingdom, he was shot. Though he did indicate that the name itself was significant.

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
The Alliance are the ancient forces that became the World Government. During the Void Century they fought against the Ancient Kingdom. Who the Alliance are and what the connection is with the Ancient Kingdom remains unknown as for now.

Birth of the World Government
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. When the research of the ponegylphs was outlawed, hundreds of scholars across the world were killed and since then a ban on any research to the void century has been put in place.

Poneglyph
For the main article, see Poneglyph

Ancient Weapons
Only 2 weapons are mentioned 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.