Warning: contains depictions of graphic violence
Each of Chainsaw Man's battles are drenched with some of the greatest gore ever to grace manga. Even though fans who've read all 97 chapters will undoubtedly agree unanimously that the most disturbing and even nihilistic moment transpires when things literally go to hell in chapter 64, there's one battle that's actually bloodier than the rest just because of how long it lasts.
In chapter 14, a group of devil hunters including Denji, the human who can transform into the series' titular monster, enter a specific hotel building as part of an ongoing effort to find the deadly Gun Devil. The building, however, is under the control of the hauntingly grotesque Devil of Eternity that keeps them perpetually stuck on the eighth floor. For example, when they climb some stairs to the next level, they find themselves back where they started. Their extended stay is so long that the devil hunters take naps at various intervals as they try to plan their escape.
After giving some time for the onset of panic to fester in the traumatized characters' rattled minds, the Devil of Eternity reveals that he will let them escape if they give him Chainsaw Man dead or alive so that he may feast on his remains. The worst part is that the devil's heart (and weakness) is nowhere near them, so they have no other choice. The giant blob that's been taunting them is merely the inside of its stomach, which gets larger the more terrified his victims get. Eventually, Denji comes up with a diabolically twisted scheme based on an equally unpleasant observation: The Devil of Eternity is clearly scared of his chainsaw and can still feel pain, so he'll just hack him up until the devil allows them to leave. Denji then proceeds to jump into the devil's mouth and tears his way out as if he were just resurfacing from a body of water. (This scene is also what spawned one of the series' most iconic images.)
Of course, there's one downside to this particular strategy. Denji's chainsaw retracts into his head once he runs out of blood. And since he's fighting nonstop, logic dictates that he'll lose the use of his brutal weapon in a matter of time. But as it starts retracting, Denji comes up with another disturbing idea: to drink the Devil of Eternity's blood. Even though it "tastes as nasty as rats," drinking the devil's blood allows Denji to continue wielding his chainsaw, transforming him into a "perpetual motion machine," as Denji calls it. Although the devil is evil, the reader can't help but feel bad for him, because this goes on for days. During this time, the devil is crying out in pain as Denji hacks away and drinks the blood spewing from the devil's wounds so that he can continue ripping into another area. Eventually, it becomes too much for the devil and he relents, pleading to die.
So while there are plenty of other exceedingly violent and overly bloody massacres, Chainsaw Man's fight against the Devil of Eternity stands out just for its longevity and how Denji extends the bloodbath to last as long as it did. This is even more impressive when taking into consideration that Denji's regenerative capabilities allow him to lose the same limbs numerous times, the frequency of which increases as the series progresses and in more brutal ways. During his face-off against Katana Man in chapter 37, Denji gets both of his arms chopped off. Denji later loses his arms again when battling the Bomb Devil who uses her explosive power to blow them off his body in chapter 51. And this isn't even mentioning the hellish event in chapter 64, when assassins from around the world come to kill Chainsaw Man, how the so-called Santa Claus can only be defeated with a terrifying ability that alters her mind, or when Denji tries to slaughter the invincible Control Devil.