The Void Century is a century-long gap in recorded and archaeological history, the study of which is forbidden by the World Government. These events occurred 900 to 800 years before the current storyline. It was hinted at during the Skypiea arc; the dates were seen in Robin's flashback. This period became known as the Void Century because there is nothing left to acknowledge anything that happened in it, 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. Though many seem to know there is a gap in the historical record, virtually everyone is unaware that filling in this gap is possible. For example, Nefertari Cobra, after being told by Nico Robin about this missing period, was completely shocked that a way to "tie the threads of history together" existed. The fact that filling the gap is possible was proven when "Dark King" Silvers Rayleigh revealed that the Roger Pirates discovered the true history. The most important event during the Void Century has to be the Great War. It ended around 722 with the fall of Shandora. In Enel's cover story, there is an ancient city on the moon with another bunch of identical Automata. There is a painting on the wall that shows a Skypiean with Automata following him on the left while a Shandorian and Birkan work on the right. After resources on the moon ran out, the three races departed for the "Blue Star".
One of the three races, the Shandorians, ended up on the isle of Jaya where they built the city of Shandora one thousand years ago. 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 Shandia, who are the descendants of the Shandorians, kept on defending the city for over eight hundred years afterwards. All three races are noted to have wings on their backs (though each has a slightly different shape) and may have even once been the same race. The connection between the sky island of Skypiea and the Void Century is the poneglyph left there for the Shandia and the Shandorians before them to protect. The Great Kingdom was a great and powerful kingdom that was destroyed by the Twenty Kingdoms, who would later evolve into the World Government. The kingdom was so powerful that its existence posed a great threat to the world, and is the reason why studying the poneglyphs is outlawed. Joy Boy arrived at Fishman Island and built Noah. He attempted to raise the Noah with Poseidon's aid, but he failed to keep his promise. He wrote his apology on the Poneglyph in the Sea Forest. After the Void Century, the Dwarves of the Tontatta Kingdom suffered oppression under the ruthless rulers of the island of Dressrosa, the Donquixote Family. However the Dwarves had forgotten what happened during the Void Century. 800 years ago, 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 Council 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 Poneglyphs was outlawed, hundreds of scholars across the world were killed. Since then, a ban on any research related to the Void Century has been put in place.At the end of the Void Century, the 20 kingdoms came together to defeat the Ancient Kingdom and then founded what is now known as the World Government. The exact process of how it was founded and how they covered up the 100 year gap remains unknown.