Sword Art Online
Sword Art Online (ソードアート・オンライン Sōdo Āto Onrain) is a Japanese light novel series by Reki Kawahara, with illustrations by Abec. It has been adapted into three manga, an anime and a video game.
Plot: A genius programmer named Kazuto "Kirito" Kirigaya enters a virtual-reality, massively multiplayer online world called Sword Art Online. There is no escape from this world unless the player clears the game; however getting a "game over" results in the death of the player. (Source: AniDB)
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Related: Sword Art Online II (Gun Gale Online)