Sword Art Online

The first MMORPG that incorporated NERVgear created by Kayaba Akihiko. Taking place on a floating castle called Aincrad, with 100 levels. It obtained infamy when the creator succeeded in trapping 10000 people in the game, and making it so death in-game would mean actual death. By the end of the death game 6000 people survived. This is the main setting for the first two volumes of the Sword Art Online series

System
The game operates similar to most MMORPGs with a level system and eventual skill mastery with some limits placed on it. The entire game is monitored and maintained by a system called <> which self corrects any problems.