In Chapter 127 we receive a small backstory about Muzan.
https://v93.mangabeast.com/manga/Kimetsu-No-Yaiba/0127-012.png https://v93.mangabeast.com/manga/Kimetsu-No-Yaiba/0127-013.png https://v93.mangabeast.com/manga/Kimetsu-No-Yaiba/0127-014.png
Muzan is given a medicine by this doctor sometime during the Heian period (794-1185). The story currently takes place in the Taisho period (1912-1926) meaning that Muzan is anywhere from 1200-800 years old. Interestingly enough the Blue Spider Lilly turned Muzan into a demon without him noticing, he only noticed right after he killed the doctor. It is also mentioned that this medicine was still in it's "experimental phase." Muzan theorizes that if he completes the Blue Spider Lilly he will be able to conquer the sun.
We know that this has been his priority since he was literally transformed. For the past thousand plus years Muzan has been searching for this Blue Spider Lilly without being able to find a hint. We also know that Muzan didn't form his Twelve Demon Moons until the early Edo Period (1603 - 1868), most likely after his confrontation with Yoriichi. https://v93.mangabeast.com/manga/Kimetsu-No-Yaiba/0155-018.png https://v93.mangabeast.com/manga/Kimetsu-No-Yaiba/0186-019.png This mean for hundreds of years Muzan only sought the Blue Spider Lily and didn't pay attention the swordsmen. It was only after Yoriichi taught them breathing techniques that he turned his attention to them. https://v93.mangabeast.com/manga/Kimetsu-No-Yaiba/0186-017.png.
Now is it a coincidence that Muzan is with Tamayo when he is confronted? We don't know how long she had been by his side at this point, but she is a gifted and brilliant doctor and the only other Demon capable of creating their own offspring besides Muzan. This stands to reason that she is extremely old, perhaps second only to Muzan in age. Muzan no doubt sought her out to utilize her medical knowledge and skills to complete the Blue Spider Lilly.
The biggest question here for me is this. Why did the doctors medicine turn Muzan into a demon? Is this not a what the fuck head scratcher moment for everyone else? Muzan is given a medicine that transforms him into a demon! Not only that but his blood is so potent that is capable of transforming other people into demons. But there also seems to be some magical/supernatural aspects to this. While the explanation of his transformation makes sense. It doesn't explain the Demon Blood Arts (which is straight up magic), or the curse the Ubayashiki family received due to Muzan becoming a demon. https://v93.mangabeast.com/manga/Kimetsu-No-Yaiba/0097-018.png
This implies that whatever the Blue Spider Lilly is, it is obviously supernatural in nature. We nor Muzan have no idea where the doctor originated from, how he created this medicine or even what the actual ingredient is. There is a big possibility that Muzan was not the first of the doctors "trials". Also given the fact that Muzan has no been able to find a trace of the Blue Spider Lilly in Japan this entire time, it may be that the Blue Spider Lilly does not grow in Japan.
The Heian Period also happens to be when Chinese influence was at it's height in Japan. https://en.wikipedia.org/wiki/Heian_period
Did this mysterious doctor originate from China? Could there be other demon progenitors like Muzan throughout the world? Many people think Muzan will be defeated this arc and the story will end. I think there is potential here to keep the story going in a way that has been forshadowed and hinted at without being seen as an "asspull." We believe Muzan to be the only demon capable of creating demons, and in Japan that's true.
Food for thought and I suppose I've rambled far too much by now.
