With the Nerve Gear, a virtual reality helmet that stimulates the user's five senses via their brain, players can experience and control their in-game characters with their minds.
All the players log in for the first time, and subsequently discover that they are unable to log out. They are then informed by Mykk Skizzy, the creator of SBO(Sequel to SAO), that if they wish to be free, they must reach the 999th floor of the game's tower and defeat the final boss. However, if their avatars die in-game, their bodies will also die in the real world.
Sword Buster Online(SBO) is the exact same game as SAO just an extended version.
Previous Games Before SBO!
Sword Art Online (ソードアート・オンライン Sōdo Āto Onrain?)
The first virtual game world, as well as the setting of the first arc of the story, known as SAO for short. The world takes the form of a giant floating castle called Aincrad, with 100 floors in it. Each floor has a medieval-themed setting and a dungeon with a boss, which has to be defeated before players can advance to the next higher floor. Like most RPGs, it implements a level-based system. However, the game is altered in a manner in which players are unable to log out, and if players die in-game, their real life body dies too.
Alfheim Online (アルヴヘイム・オンライン Aruvuheimu Onrain?)
The setting for the second arc of the story, known as ALO for short. All players in the game have wings and are capable of flight. It is a large world, divided into separate 'homelands' for each of its fairy races. In Alfheim's center is a very large tree called the World Tree, and the goal of the game is to reach the top. It implements a skill-based system with players increasing their stats by developing both their combat and non-combat skills. Aincrad, the castle of the first game, is later accessible to ALO players as well.
Gun Gale Online (ガンゲイル・オンライン Gan Geiru Onrain?)
The setting for the third arc of the story, known as GGO for short. It is a virtual game world with a main focus on guns, although melee weapons like lightsabers and knives also exist. From all the games it is the most competitive one as the money earned there can be exchanged for currency used in the real world, drawing high-tier professional players to make a living from it.
UnderWorld
The setting for the fourth arc of the story. According to Kirito, it is graphically the most realistic of all VRMMOs to date. The flow of time in UW is variable and can be much faster relative to the real world's. Even so, it seems that even Kirito was misled as to the actual rate of UW's time flow, so it is still unknown. However, in the as of yet canon "There is but one ultimate way" side story, the rate is said to be safely up to 1,000 times faster than that of the real world, just like Accel World's Brain Burst.
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Sword Buster Online, Is taken back to The Original, Sword Art Online. Though it is by a different creator, it doesnt have many differences except the fact that though you're able to log in an play the game. The game has not been finished.