As Tanjirou lies in the rubble, bloody and exhausted, he’s briefly transported back to his snowy home, where a pre, then post-demon Nezuko tells him to stop blaming himself (as if) and keep fighting. The vision is at least enough to wake him up, but he’s almost sorry he did.
That’s because the scene around him is a hellscape of flames, and among his allies, he’s the only one still conscious. Gyuutarou spends a good half of this episode taunting him and laughing at how “disgraceful” he is, especially to have to be protected by his demon little sister. It’s a welcome change of pace from the unrelenting battles of the past few episodes, with no score and only the flickering of flames accompanying the dialogue.
When Gyuutarou eventually falls, he’ll regret wasting so much time toying with Tanjirou, who was simply playing possum and waiting for a chance to stick a poison kunai in his opponent’s leg, then using Hinokami Kagura to attempt to slice off his head. When Gyuutarou counters with his blades, they’re deflected by Zenitsu, who while still asleep is back in the fight.
As Zenitsu continues to battle Daki, Tanjirou is bailed out by Uzui, who stopped his own heart to keep the poison from getting there, and can also double-wield even with just one hand. Daki, with his second wind, goes in for the win, having charted the musical “score” of Gyuutarou’s moves. Tanjirou keeps up as best he can as Uzui and Gyuutarou exchange slashes with increasing speed and ferocity.
When Tanjirou jumps into attempt another beheading, Gyuutarou stabs him through the jaw with his poison blade, meaning there’s just that much less time for Tanjirou to fight before succumbing that poison. Still, Uzui has him where he wants him, and Gyuutarou’s neck is exposed, so Tanjirou finds yet another higher gear, his scar seemingly spreading across his face in a flame pattern.
After a lot of screaming, Gyuutarou’s head is finally sliced off. At the same time, Zenitsu, with help from Inousuke (who moved his heart so it didn’t get impaled) slice off Daki’s head. The two siblings’ heads fly gracefully through the air, then fall to the ground, bounce, and roll to a rest right beside each other.
The battle is seemingly over, and the Demon Slayers have won…but then Uzui shouts “RUN!”, for Gyuutarou self-destructs, enveloping half the district in a massive fireball. As those flames die down the credits roll with a certain finality, until the post-credits omake is handled by pre-demon Nezuko.
Assuming Gyuutarou was destroyed in that blast, the question becomes who survived. Obviously Tanjirou, Nezuko, Inousuke, and Zenitsu. I find it hard to believe two straihgt arcs would end with the death of the Hashira, and the three wives haven’t quite gotten enough development (or screen time) for their deaths to make much of an impact…so we’ll see.