* The following contains MAJOR SPOILERS for Shaman King.
The tournament arc is a staple of shonen storytelling. It offers creators a chance to pit characters against each other who wouldn't necessarily have cause to fight otherwise. In doing so, the fundamental identities of the characters can be not only drawn out but compared, questioned and challenged. Some of the best fights in manga/anime come from tournament arcs, but no tournament has ever been as brutal as the one presented in Shaman King.
For those unfamiliar with Hiroyuki Takei's cult classic manga and subsequent anime adaptation, here is a brief refresher. Every 500 years a tournament is hosted by the Patch Tribe that pits spirit-wielding shamans against each other. The grand prize is to merge with the Great Spirit. The manga, in fact, lists Jesus, Muhammud and Buddha as previous winners of the tournament. This means that Shaman King doesn't simply have a tournament arc, Shaman King is a tournament arc -- one in which the winner is crowned messiah and gifted the ability to reshape the world in their desired image.
Of all the various iterations of the classic tournament arc there are out there, fighting your way into the position of literally becoming God -- as is the ultimate goal of the series protagonist Yoh Asakura -- is far and away the loftiest. Not only does the series protagonist hold a seemingly impossible goal, but so does every other competitor that he and his friends come up against throughout the story. Adding to this is the fact that these competitors have been haunted by ghosts for their entire lives, as well as reviled by societies who see them as insane at best, and at worst, vile representations of sinfulness and evil, and the stage has been set for some unbelievably brutal combat.
Take for example Faust VIII, a man who wields the soul of his deceased wife and whose ultimate move consists of compiling thousands of skeletons into one giant skeleton that he uses to crush his enemies among a pile of corpses. A fun fact about Faust is that when fighting the protagonist in the manga, Yoh destroys the bones of his wife's legs (he carries her skeleton with him under a big coat, of course) and he literally replaces them with his own surgically removed leg bones. Or Ren, the Chinese Shaman who, in his qualification match, successfully landed the one hit necessary to secure him a spot on the bracket but instead, opted to kill the tournament official before laughing at the man's lifeless body.
It's notable that both of these characters very quickly join Yoh's team and are seen as heroes. All of this, of course, pales in comparison to what we see from the series main antagonist and Yoh's sort of twin brother, Hao/Zeke Asakura.
Hao is Yoh's twin brother, but he's also a 1500-year-old shaman with the power to choose his reincarnations, hellbent on becoming the Shaman King so that he can eradicate all human life -- save for shamans. He views this as an act of revenge for his mother. Hao murders people with such little remorse on so many occasions that it would make Meruem raise his eyebrows. At one point he casually immolates three people with a smile before walking out of the shaman battle arena.
All of this is nothing compared to what happens when Hao wins the tournament. Yes, that's right: the antagonist wins the tournament and becomes God. Upon doing so, he kills off all of the members of the Patch tribe with a single glance and then proceeds to eliminate the population of an entire continent. When Yoh and his friends attempt to stop him, Hao creates a supernova to kill them, which, in turn, creates a black hole that eliminates everything save for him and the Great Spirit. It is only in the split second before the end of everything that Hao's mind is changed by communicating with the spirit of his dead mother and all of the people he has killed -- on a conditional basis.
Normally, tournament arcs occur near the beginning of a shonen series and so the stakes are relatively low. The one obvious exception, and the only one that comes anywhere near this one, is Dragon Ball Super's Tournament of Power; even then, the carnage isn't nearly as wholesale or gruesome. Shaman King is not without its flaws. If the description of its ending seemed insane and rushed, that's because it unfortunately was. Even with that in mind, you'd be hard-pressed to find a shonen tournament as brutal as this one.
