Xaos is the main antagonist of Season 8. A long time ago, he helped Master Hand create the Smash World, but once Master Hand found out Xaos was a infamous murderer, he locked him away in Pandemonium. However, Xaos had put an Imperial Seal on Master Hand, making him his slave. Xaos now desires the keys to Pandemonium so he can gain access to the core of Pandemonium. He has Team Shadow, consisting of Venus, Leon, Shade, Blaze, and Flare do this for him, though why he can't do it himself is unknown. So far he has acquired 11 of the 12 keys to Pandemonium, 4 of them having been dropped while Master Hand was escaping Pandemonium, Venus having acquired Sasuke, Goku, and Ren's keys, Shade having acquired Psycho Kirby's key, and shadows of Sasuke, Goku, and Bowser having acquired Sonic, Fabia, and Zombie Spider-Man's key. Like Naruto, Xaos is on the sidelines for most of the season. In the finale, Naruto, Sasuke, Sonic, Goku, Zombie Spider-Man, Psycho Kirby, Fabia, and Ren finally battle against Xaos. In the end, Xaos is defeated and imprisoned in the middle dimension. He is freed prior to Season 10 and teams up with Zombie Reed, Mag Mel, and Smithy. Xaos still desires the core of Pandemonium, and is infuriated that it is being used by the Heroes of Legend, who he considers vermin. Xaos is finally killed when Lena breaks his neck with her mind. Xaos is revived offscreen prior to The Keys of Smash Arc, where he teams up with Yami Bakura and many of the villains to gain a mysterious key the heroes acquired. Xaos later returns in Season 22, where he and several of the villains attack the Heroes of Legend while they are guarding the Mind Chamber as Samus Aran is inside. Xaos also reveals that he has been working on a powerful transformation ever since he was defeated by the Heroes of Legend. Later, it is revealed that Xaos was innocent, with Master Hand being the true villain, and Xaos' actions were because he felt only he could destroy the Hands.
- Xaos is the first villain to have the same model as another character in the series. Xaos has the same model as Ganondorf.
- Since Team Shadow's 2nd transformation is on par with Xaos' base power level, that may mean Xaos is very powerful.
- Xaos can brainwash people to do his bidding, which he had done with Sasuke, Sonic, Goku, Zombie Spider-Man, Ren, and some of the heroes who were defeated by Team Shadow. How he does this is that he transfers his energy into the person he wants to control. However, it can be broken, since it has been done with Fabia, Kirby, Mr. Game & Watch, Psycho Kirby, and Dr. Mario.
- Xaos is the second most heartless villain in the series (falling behind Yami Bakura), since by brainwashing Sasuke, Zombie Spider-Man, Psycho Kirby, Zombie Wasp, and Zombie Giant-Man, he had undone their redemptions, even though they are free now.
- Xaos is very dangerous, as he had a strong grasp on Sasuke and Vegeta, who are normally impossible to control due to their pride.
- Unlike Mecha Sonic, Zombie Reed, and Sasuke (when he was an antagonist), who fought against the heroes many times in the series, Xaos didn't battle until the finale of Season 8.