I'm just wondering about the Void Century in One Piece. What happened during those years? The World Government came to be by taking down the mysterious KINGDOM. I really want to know the name of that kingdom. Can anybody guess or give me something that would put me at ease?
The events during the Void Century has not yet been revealed, so for everyone it still remains a mystery, except for the Roger Pirates who have read the Poneglyphs and discovered the true history which the study is forbidden by the World Government.
As for the name of the Ancient Kingdom, its true name is a mystery, as the Gorosei ordered that Professor Clover be shot before he could speak its name.
The only details revealed so far are:
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 800 to 900 years before the current storyline. It was hinted at during the Skypiea arc; the dates were seen in Robin's flashback.
Birth of the World Government 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.
Ancient Weapons The three Ancient Weapons are three weapons of mass destruction that are powerful enough to bring ruin to the entire world. They are known as Pluton, an ancient ship capable of massive destruction, Poseidon, a Mermaid with the ability to communicate with and control Sea Kings, and Uranus, the properties of which remain unknown. The Ancient Weapons are the argument used by the World Government to forbid the research of the missing years.
You can read more about it here.