Skeith

Skeith is a powerful weapon with the potential to break the entire game of Sword Art Online. It is a central item in Sword Art Online: Clashing Blades and Sword Art Online: Last Light.

Background
Skeith is the result of a glitch in the coding of Sword Art Online that allowed for numerous game breaking glitches to enter the game. Kayaba Akihiko quickly noticed the glitch and fixed it, but not before a piece of the corrupted coding was released into Aincrad. This small bit of data is able to perform numerous game breaking acts when wielded by a player. It is not a hacking method, as thought by many, but simply a weaponized glitch used to alter the game in numerous ways. The first player able to control Skeith was Majora.

Properties
Skeith has no definitive appearance, simply appearing as a cluster of data drifting around Aincrad in any place, on any floor, at any time. It is also very difficult to see as it moves along with the non corrupted data of the game itself. Any player attempting to control Skeith runs the extremely high risk of their player data being corrupted and deleted, therefore killing the player character and the player in real life, and simply being near the corrupted data cluster will cause the player's data to slowly glitch out and corrupt. Containing Skeith is possible, however, and players who manage to do so are granted mastery over the game in very much the same way the Game Master himself does, although since it is corrupted data it will also corrupt the entire game if large scale feats are attempted, which can crash the game and kill everyone in it. While contained, Skeith appears in the player's inventory and skill list in a corrupted inventory space or skill spot, which serves as its container. Skeith itself in this state has no physical form, and is simply used directly from the player's inventory or skills.

With each successful use of Skeith, player data will slowly be corrupted little by little. Although the game will recover the corrupted bits of data as it is supposed to, excessive use of Skeith will permanently corrupt user data, and lead to the player's deletion. The safest time is at least a month between uses.

It has been shown by Majora that pure willpower alone can prevent player deletion and death. However, the corruption will still take effect - albeit at a much slower pace - and can lead to extreme mental instability and insanity.

Abilities
As noted, Skeith has the potential to permanently corrupt and crash Sword Art Online, as well as performing game breaking acts for the player controlling it. However, if a player attempts to alter their player data to, for example, grant invincibility or level themselves up, Skeith will corrupt the player's data and in effect kill the player, making these alterations impossible. However, player data can be changed to some extent indirectly (ie. altering an item that gives certain buffs to a player), although it is still dangerous, as either the item will be corrupted and erased or the corruption on the item will spread to the player and both will be erased. There is no real way to learn how to use Skeith, and everything done with it is based on chance.

As a weapon, Skeith has the ability to completely destroy NPC or PC data instantly, ignoring all variables, erasing everything about the player or NPC from the game. This allows for easy kills, but also deletes the experience and money value, as well as any drops it may have. Further, any unique NPC enemy destroyed by Skeith will never respawn, as the data of the enemy itself is erased along with the avatar of the enemy. The Game Master is not excluded from this.