Demon Slayer: Infinity Castle is one of the best anime movies of all time, and there's a villain in the film who might've taken the franchise to another level. Demon Slayer has only ever had one ironclad rule: to kill a Demon, a Demon Slayer has to decapitate them.
It doesn't matter if a Demon is sliced to bits, has their limbs chopped off, or is crushed brutally. If their head is still intact, they can regenerate any damage they've sustained. It's one of the major reasons why Demons are such a threat in the franchise, and one Demon, somehow, broke that single rule.
Demon Slayer: Infinity Castle showed that one of its major villains was truly in a league of his own. While Muzan Kibutsuji is certainly the central antagonist of Demon Slayer, even the King of Demons has nothing on this villain.
Demons Have Always Been Killed by Decapitation
Every Demon Has Died This Way, Except for One
Demon Slayer Hashira Kyojuro Rengoku takes on Upper Level Demon Akaza
Demons are no laughing matter. Even mindless, unthinking Demons are stronger than the average Demon Slayer, forcing members to team up just to take a single enemy down. There are plenty of ways to damage a Demon, but only one way to end them permanently: to behead them.
Every Demon in the series so far has been killed this way. From the Spider Demons early in the series to the Upper Rank Six Gyutaro/Daki combo in one of the best fights in anime history, once a Demon loses its head, it's over.
Demon Slayer: Infinity Castle proved that even a headless Demon is still a problem. Not only did a certain someone survive decapitation, but he began to regrow his head after it was sliced off, cementing himself as one of the best villains in anime in the process.
Akaza Is the Sole Exception
He's So Strong He Started to Grow His Own Head Back
Akaza from Demon Slayer Infinity Castle
Akaza, the third-strongest Demon under Muzan, broke the only rule in the entire franchise during his explosive fight against Tanjiro and Giyu. After Tanjiro entered the Translucent World, he was able to behead Akaza in a swift strike. Ever the fighting god, Akaza's headless body continued the battle while his head began to grow from his neck.
Akaza is such a powerful villain that the only character that could destroy him was himself. After regaining his memories, he decided to decimate his own body and grant Tanjiro the victory that he felt the young Demon Slayer earned. It was an emotionally cathartic moment that made Akaza the best villain in the franchise.
The Upper Moon is not only one of the best-written villains in anime, but he's one of the best-written characters in general. It's almost disappointing that he won't be seen in Demon Slayer: Infinity Castle Part 2, but his ending was about as good as it gets for an antagonist.
