Warning: Spoilers ahead.
The Mugen Train derails. All that gross gooey purple stuff actually acts as a cushioning for a lot of the train. So somehow all the passengers survive. Although, many are injured.
Tanjiro is flung from the train and can’t even get up. He’s in rough shape. Inosuke checks on him, and Tanjiro asks him to help the conductor who stabbed him. Inosuke doesn’t want to, but Tanjiro says he’s suffered enough. So they should help him. Inosuke begrudgingly agrees to.
The last living blob of Enmu starts to disintegrate. He can’t believe he lost. He blames all the Demon Slayers for stopping his plan. Especially Tanjiro. He dies with frustration on his last breath. Good.
Tanjiro uses Total Concentration to try to stop his bleeding. But it takes some more advice from Rengoku before he can finally do it. He makes his breathing more precise, and stops the bleeding.
Rengoku says now Tanjiro can rest. But the peaceful moment and happy ending are broken up by the unexpected arrival of a new demon. Tanjiro can tell from the demon’s eyes - this is an Upper Rank Three demon! It has the strongest scent of Muzan that Tanjiro has ever smelled on a demon. In other words, really bad news. A really powerful demon.
Right away the demon lunges to kill Tanjiro with a punch. His fist gets close to Tanijro’s face, but Rengoku is able to slice up his arm and stop him.
The demon doesn’t even feel pain. He regenerates instantly. He introduces himself as Akaza.
Akaza asks Rengoku to become a demon. He says humans are weak and all eventually die. He admires Rengoku’s strength. He wants Rengoku to live forever as a demon, training with him to get stronger and stronger for hundreds of years.
Rengoku declines. He says mortality is what makes humans so amazing.
The two get into what’s probably the most high level fight that’s ever happened in the series thus far. Inosuke realizes it’s too fast for him to get involved - instead of helping Rengoku, he’d only be a hindrance to him.
Rengoku cuts off Akaza’s arms many times with his Flame Breathing techniques. But Akaza regenerates instantly and doesn’t tire at all. Finally, Akaza’s blows connect. Just a few hits, and Rengoku is seriously injured.
Tanjiro is watching the whole thing. And he’s seriously worried about Rengoku, who is now beaten up!
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