Akaza certainly added a lot of suspense to the Mugen Train movie. If you were wondering why he was there in the first place, however, you’re not alone. The answer is pretty simple:
Akaza came to the Mugen Train because Muzan ordered him to. Muzan wanted to get rid of both Rengoku and the other Demon Slayers (mainly Tanjiro) and knew Enmu likely wasn’t up to the task. That is why Muzan chose to send Akaza, the Upper Demon closest to the train.
This information is revealed in episode 1 of the Entertainment District arc, which takes place right after the Mugen Train arc. Muzan, looking quite differently since last we saw him, scolds Akaza while revealing interesting information to the viewer:
Why did Muzan send Akaza to the Mugen Train?
Muzan is always on the lookout to eliminate Demon Slayers. Having four of them in one location, including Tanjiro and a Hashira, makes the Mugen Train a clear target for him. Knowing Enmu likely isn’t up to the task, Muzan sends the nearby Akaza to finish the job.
As Akaza fails to finish them all off before the sun comes up, Muzan agrees with Tanjiro that Akaza actually lost the fight. Muzan torments Akaza for several tense moments before dismissing him, with the latter vowing revenge against Tanjiro.
Why does Muzan hate Tanjiro?
There are several reasons for this. The first two reasons are only minor spoilers if you’ve been following along until Mugen Train, but the last one is a big one. So if you don’t want to get spoiled too heavily, I recommend you stop reading this page after the first 2 points.
- Tanjiro has run into Muzan before and is potentially able to track him down. Although Muzan now has a different appearance, Tanjiro can make use of his excellent sense of smell to recognize him. As Muzan reminds Akaza, his goal is never to have another Demon Slayer enter his line of sight again.
- Nezuko is in a state of being that shouldn’t be possible according to Muzan. Demons should not be able to collaborate with humans. Nezuko is out of his control and Tanjiro is the one who managed to snap Nezuko out of it.
As I wrote above, the third point is a big spoiler.
If you want to turn back, you still can, but this is your last chance.
….don’t say I didn’t warn you.
3. Tanjiro’s earrings mark him as being part of the same lineage of the only Demon Slayer Muzan truly fears. As clarified by Rengoku’s father in the first episodes of the Entertainment District, this lineage invented all breathing techniques. All other breathing techniques are a (weaker) derived form of these original breathing techniques. This is also the reason he commands Enmu to hunt down “the Demon Slayer with the sun earrings”.