In the last episode of "Chainsaw Man", Aki defeats the ghost devil when he remembers that it is blind and can only sense fear. Recollections of Himeno allow him to approach it, fearlessly, and he decapitates it with his samurai sword.
Meanwhile, Chainsaw Man and Katana Man have their final showdown on a train. Katana Man looks to have won when he cuts off both Chainsaw Man's arms, leaving him with only the blade growing out of his head. Unsurprisingly, though, he still manages to get the upper hand somehow.
We then see Katana Man returned to his normal human form, tied up in his underwear, and Denji convinces Aki to have a competition where they compete to see who can make him scream the loudest by kicking him in the groin.
Remember how Katana Man's face melted off when he transformed back into his human form? I don't think they really explained that. Does Chainsaw Man do that when he transforms back into Denji? I don't remember. And how do they transform back? Denji pulls the ripcord when he wants to become Chainsaw Man. Does he pull it again to become Denji?
There's a lot of unanswered questions, but maybe that's part of the point. The show is chaotically insouciant in regards to making sense, and therein lies its charm. It's insane, violent, unpredictable, occasionally hilarious. It's one of the better anime shows I've seen.