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[LEVEL 5 CLEARANCE APPROVED]
Skeith was created within the game of Sword Art Online as pieces of broken code. It is essentially an infectious glitch that managed to escape initial testings of the game as beta tester Takeo Tohno found it, but chose not to report it, thereby allowing it into the main release of the game. It would eventually be claimed by Takeo in the main game.
Skeith is essentially lines of broken code made manifest in the game, and is therefore capable of causing dangerous game-breaking anomalies within Aincrad. Its effects are exacerbated when wielded by a compatible player, capable of feeding off of their emotions and amplifying them, turning the player in question into the worst possible version of themselves. It also allows the player to feel pain, and essentially makes their in-game avatars physical and capable of sustaining damage, but also allowing the wielder game-breaking power.
Skeith, however, has a deteriorating effect on its user, as it exacerbates their darkest and most negative emotions and thoughts, eventually overriding their entire personality to be that and nothing else. This was shown with Takeo Tohno, whose consciousness was completely overwritten by Skeith until he himself fell into a coma and eventually died, while Skeith itself assimilated his in-game body and replicated his personality, becoming a physical manifestation of Skeith.
This was further demonstrated in Skeith's second user, Ishi Haseo, where his already unstable personality became more exacerbated over time, though at a slower rate than Majora, indicating that some level of willpower is capable of slowing the deterioration, as well as certain outside factors such as the presence of others for emotional support, this role fulfilled for Ishi by Misaki Akimi and then later Asuka Shizuka. Nevertheless, Ishi very nearly became the second manifestation of Skeith Avatar, only stopped by Yamato Ovan wielding Corbenik. Ishi's willpower and the people supporting him would eventually completely stop the deterioration of his sanity by Skeith, allowing him to utilize it as a powerful weapon with minimal detrimental side effects to himself, the worst of which being nearby data becoming unstable and causing temporary localized glitches.
Majora - The first user of Skeith in Sword Art Online, and the one responsible for letting it into the game to begin with. Majora initially gained great power in Skeith upon first obtaining it, but it would slowly deteriorate his psyche and bring about his more violent tendencies, exacerbating his jealousy of Yamato and his desire for power more and more until that was all that was left of him. He would eventually be assimilated completely by Skeith, falling into a coma in real life and dying soon after, his body and mind manifesting as Skeith Avatar. It would be defeated and destroyed by Yamato with the use of Corbenik.
Ishi Haseo - The second user of Skeith. Ishi would obtain Skeith and have very much the same effect on him as it did to Majora. The only difference being Misaki Akimi, who acted as a stabilizing force for Ishi, greatly slowing Ishi's descent into madness and allowing him to utilize Skeith to a greater degree than Majora had been able to, though he would be reluctant to ever use it, coming to realize its effects on his mind. Afer Misaki's death, however, Ishi becomes unable to resist Skeith's influence and it temporarily takes him over, only being stopped in the end by Yamato using Corbenik to suppress Skeith within him. Yamato would train Ishi in the ways of moderation and patience through instructing him in the Kyosho Style of kenjutsu, and along with the support of Asuka Shizuka, Ishi comes to tame Skeith and his dark nature. He would eventually also acquire Corbenik, utilizing both together against Hao and his forces during the final hours of Sword Art Online.