Compiled as much as possible in chronological order from canon, Taisho Secrets, and fanbooks, which a few general facts thrown in for additional interpretation. Also, a little comparison of the amounts of humans consumed by different demons. Please feel free to let me know if I’m missing anything or made any mistakes.
We begin with Muzan’s birth in the Heian Period…
It’s the Heian Period (794-1185), and since Muzan is side to be over a thousand years old, I’m placing him born around the year 900-ish.
Muzan is born into what would be the future Ubuyashiki Clan, likely in the Heian capital (modern day Kyoto). Assumed stillborn, he is barely rescued from cremation (a sign of being born very high class nobility).
Muzan grows up sickly and has never had much regard for other people, seeing them akin to bugs, and from around the time he was human to shortly after turning into a demon, five of his wives committed suicide due to his harsh tongue.
A doctor who lived around Mt. Kumotori in the Kanto region (modern day Tokyo area) uses the blue spider lilies that grow there to try to create a medicine to treat Muzan.
Muzan, thinking the medicine isn’t working, kills the doctor partway through treatment. It works, but he is no longer able to go in the sun. No qualms about satisfying his cravings for blood, but he is really ticked out about the possibility of the sun killing him.
The Ubuyashiki clan’s children keep getting ill. They are told their family carries a curse for Muzan having come from their bloodline, they swear to eliminate Muzan.
No one is safe when born into the Ubuyashiki name (which may have been chosen with origins in ancient Shinto mythology, perhaps with reference to no matter how many of them you kill, they will stubbornly keep birthing more). Girls must marry out and change their surname by age 13, or no matter how careful they are, they will die young by illness or accident. There is only ever one surviving boy per generation, the others all die.
Heian period ends, the Genpei War (1180-1185) sweeps Kyoto and many other parts of Japan into violence, over the centuries thereafter the seat of power switches from Kyoto to Kanto (it’ll switch back later for a while). At some point Muzan has moved to Kanto (likely in search of the blue spider lilies, but possibly because Muzan likes being among those in power too). Swordsman in the Corp do not yet use Breaths and fight in whatever method they can, at some point they began using Nichirin blades because the ore soaks the sunlight. While their sword forms are already in reference to the basic Breath forms we’d know later, they are not yet using Breath techniques. The Pillar system is already in place.
(Rengoku family already practices Kankagari, hence the hair color.)
We get a little fuzzy here. Generally then Tanjiro and company make reference to Yoriichi’s era, they say “over 300 years ago” but according to Kokushibo’s recollection, he last met Yoriichi 400 years ago. That makes us a little too early for the Warring States/Sengoku period, defined as 1467~1615. It’s also possible Kokushibo was rough in his estimation, so I’m splitting the difference and putting them at early Sengoku, giving Kokushibo 370 years since last meeting Yoriichi.
The Onin War occurs 1467-1477, plunging Japan into the Warring States period as it is more technically defined, so we can assume a lot of the important events in the twins’ early life take place in this period of chaos. I’m assuming based on the demon attacks at Mt. Kumotori that they, Muzan, and the Ubuyashiki clan are already based in the Kanto (future Tokyo) region, but a lot of the early, heavy violence of this war took place in Kyoto.
Around the early 1460s, twins are born to the Tsugikuni Clan. Their father, worried about succession disputes, wants to have the second-born twin with a flame birthmark killed (in order to avoid some tragedy that may strike their clan before they are the age of 2), but their normally very quiet mother Akeno puts a stop to that. She names that baby Yoriichi so that he may value relationships, their father names the older one Michikatsu to value victory.
The boys are raised in separate conditions, the illness on Akeno’s left side slowly progresses. Michikatsu feels bad for Yoriichi and brings him presents, including a handmade flute that doesn’t work well. For years Yoriichi is quiet so as not to trouble his father, so he is assumed deaf. Akeno gives him the earrings when he is seven and prays to a sun god to open his ears, at which point he says his first words, informing her that he can hear. Michikatsu is beat by his father for going to play with Yoriichi, which Michikatsu insists he doesn’t mind, but this leads to severe arguments between their parents.
Still the age of 7, Yoriichi has been watching Michikatsu’s sword lessons and is allowed a shot at one of their father’s retainers, whom Yoriichi very swiftly defeats. Yoriichi has little interest in swordsmanship but because Michikatsu asked, he explains how he sees the see-through world. Michikatsu’s jealously begins.
Because the retainer was so impressed, Yoriichi and Michikatsu’s roles change completely, Michikatsu is then the one fated to be sent to a temple at the age of 10 to follow a monk’s path. When they are 8 (1470, give or take), Akeno dies at the age of 24, Yoriichi says farewell to Michikatsu, and says he’s going to the temple.
Their father loved Akeno very much and he becomes distraught, for he never knew of her illness. As it was her wish to have the boys raised as equals, he tries to amend his ways and goes to the temple to try to bring Yoriichi back, but Yoriichi is not there. (To Michikatsu, this looks like their father is desperate to bring back the more powerful heir.)
Instead of entering the temple, Yoriichi runs a very far distance and encounters an orphan named Uta. They soon become traveling companions, and it’s thanks to Uta that Yoriichi learns his ability to see the see-through world is not normal.
10 years later, (roughly 1480, Yoriichi is age 18), Yoriichi and Uta wed and settle on Mt. Kumotori.
By or around this time, Michikatsu is also wed. Their father dies shortly afterward at age 35. Michikatsu goes on to have 2 children.
When Uta is soon to give birth, she is killed by a demon and when Yoriichi finds her, he stays by her corpse for about 10 days. He is found by a Rengoku ancestor, who invite him to join the Demon Slayer Corp. (By the Taisho period, blue spider lilies grow at the spot where Uta was buried. He never falls in love again.)
Yoriichi begins teaching Breath techniques to other Corp members, their demon fighting abilities rapidly get stronger. He gets along very well with the Pillars, who improve their swordsmanship rapidly. They cannot quite manage Sun Breathing, but make the five basic off-shoots: Rock, Water, Wind, Thunder, and Flame. Rengoku keeps many records of this time which are handed down throughout his family.
Yoriichi rescues Michikatsu from a demon. Michikatsu is so taken by Yoriichi’s Sun Breathing that he abandons his family to enter the Corp and dedicate himself to learning that technique. He does not pick it up, but develops Moon Breathing. He attains a mark like Yoriichi’s, and many other slayers have attained marks as well. Michikatsu is concerned about being the best and passing on his legacy, Yoriichi still doesn’t care about these things.
It’s very likely that Tamayo is a demon by this time.
Muzan lays low a while, but Michikatsu brings him the head of Oyakata-sama (it is unclear from Gotouge’s notes whether this was immediately upon his becoming a demon, or if it was a few years later and this is the head of the child who oversaw Yoriichi being exiled). The Corp henceforth enacts much stricter secrecy, and this is also around the time Kakushi are organized.
The young Kamado newlyweds, Sumiyoshi and Suyako, come across Yoriichi’s empty former home on Mt. Kumotori (early 1480′s-ish now). It’s a little run down, but they tend to rescue a lot of people who try to pay them back later. As they refuse the money, the people insist on fixing up the house for them. (More specifically, they rescued a Daimyo’s wife and her son who went into hiding in the mountains when they were under attack in a succession dispute.)
The young Kamado couple (including a very pregnant Suyako) is attacked by a demon, but Yoriichi, who happened to be passing through and wanting to stop by the place he lived with Uta, rescues them. Yoriichi is also the one who fetches a midwife (extremely quickly) for Suyako to give birth to Sumire. Shortly after Sumire is born the following day (in early spring), Sumiyoshi already insists that he’ll tell future generations about Yoriichi, but Yoriichi sternly tells Sumiyoshi that he hasn’t been able to save anyone, and not to see him as any kind of special person, and disappears. ***THIS MAY TAKE PLACE LATER.***
Those with marks begin to die off by the age of 25 (according to ancient practices for counting age, this may have put them around age 23 or 24 as we think of it today. This would also have made Yoriichi around age 6 when he beat the retainer and left home). Michikatsu is distraught, and Muzan approaches him, declaring that he is curious about what will happen if he makes a demon out of a Breath user, and giving him the choice to live on forever or die early because of his mark. Michikatsu choses to become a demon, and he maintains a loyal and open relationship under Muzan ever since. This is likely still around mid-1480s, or roughly two years after Yoriichi saves the Kamado family (if this indeed happened in advance).
Shortly after that, Yoriichi encounters Muzan, Muzan declares he no longer has any interest in Breath users (after all, he didn’t master the sun, dang it). Yoriichi feels he was born to defeat this man, delivers a stunning display of Sun Breathing, but the little pieces of Muzan that get away altogether form roughly the size of a head. Tamayo is freed from Muzan’s curse and tells Yoriichi as much as she can about Muzan, including that he probably won’t appear again in Yoriichi’s life. Yoriichi lets her go.
Yoriichi is then informed by Corp members (including Rengoku) that Michikatsu has become a demon.
This, in addition to Yoriichi failing to kill Muzan (give him a break, guys) and letting Tamayo go, makes some Corp members demand Yoriichi’s suicide to atone, but the 6-year-old Oyakata-sama (whose father is still alive, but likely incapacitated) puts a stop to this. Yoriichi is exiled from the Corp and told to kill his brother. Some members of the Corp (in particular, Rengoku) stay in touch with Yoriichi with Oyakata-sama’s tacit permission.
After being kicked out of the Corp, Yoriichi visits Sumiyoshi one fall, wanting to tell someone about his life. After sharing his story, he receives flowers from Sumire, and at Suyako’s request, he performs Sun Breathing. Sumiyoshi memorizes it by watching, and promises to pass it down. Yoriichi leaves him with the earrings his mother Akeno gave him, and keeps the toy flute from Michikatsu and a scrap of Uta’s kimono.
***It’s also possible Yoriichi’s first visit back to Mt. Kumotori was after explusion from the Corp, and it was after two years of being out on his own that he sought out Sumiyoshi again.***
In 1545-ish, Yoriichi is over 80 years old and finally finds Michikatsu/Kokushibo. Yoriichi barely fails to kill him and Breaths his last. Kokushibo cut the flute in half when cutting Yoriichi’s standing corpse, but keeps the pieces with him.
After Yoriichi’s death, Muzan goes on the offense again, eliminating anyone with knowledge of Yoriichi’s Sun Breathing techniques. The Corp is essentially erased for a time, and thereafter, they no longer find themselves as strong as they were in Yoriichi’s time. According the Rengoku records, this is because there was never any teacher as good as Yoriichi after that.
Over two hundred years pass of Muzan trying to find the blue spider lilies or create a demon that can master the sun, and swordsman polishing their Breathing techniques as they slay demons. By this time we can assume most of Muzan’s activity is based around the Kanto area in the bustling Edo period. (Fun aside, lots of popular kabuki plays would be developed with themes of demon slayers. Also, based on his tastes and fashion choices, Muzan likes to be among the affluent and influential people of society.)
Around the later decades of the Edo period, Muzan gets the idea to make the Twelve Moon Demons with a large amount of his blood. Hantengu might had been one of these first experiments, and Akaza, meh, I’m putting his formation any time between 1785 and 1800, put that’s really loose.
Douma’s formation was after this (and he probably got busy eating lots and lots and lots of nutritious humans right away) and Gyutaro’s still later. All of the Upper Moons were in place by no later than 1802, as they have not been called to the Infinity Fortress all together in 113 years.
My assumption is that once Muzan got the idea, he went around and made the Upper Moons in a very short period of time, and Kokushibo likely already had a lot of Muzan’s blood in his “Maybe a Breath user will master the sun” experiment. There isn’t much indication if the Lower Moons were also formed around this time, or if they came later on a second whim.
The following hundred years is full of a cycle of Pillars and the Twelve Moons pit against each other while the common riffraff fight the common riffraff. While the Pillars kill Lower Moons as a rite of passage and they are constantly changed out as openings occur, the unchanged Upper Moons routinely kill and eat Pillars. Even at the bottom rung, Daki has eaten 7 and Gyutaro has eaten 15. For each of those Pillars, at least one Lower Moon or fifty demons has died.
As a point of comparison, the Hand Demon has been a demon for roughly the same amount of time, or a few years longer (he was locked away on Mt. Fujikasane around 1868). Although not granted any special amount of Muzan’s blood like the Lower Moons, it’s nursed a grudge and eaten 50 children (13 of which were Urokodaki’s), surviving and growing much, much powerful than most of the demons on the mountain who had only eaten 2 or 3 humans. On that note, former Lower Moon Kyogai seemed to find it difficult to consume any more humans, and wanted to consume Marechi (rare blood) to achieve the equivalent of eating 50 humans. While Akaza is allowed to be picky and focus on training, I have to wonder if it is generally a difficult part of many demons’ existence to have to consume human flesh.
But I digress.
You know who doesn’t care about that? Muzan.
If we take 1915 as the year Tanjiro’s Final Selection takes place and assumed modern age counting methods as opposed to the standard used until Meiji (which was still often used in Taisho), then Ubuyashiki Kagaya was born around 1892-ish. His father couldn’t take the sorrow of being in that position, so when he committed suicide at the age of 19, Kagaya assumed the role of the 97th generation Oyakata-sama at age 4 (his other siblings, all brothers, had died). He very soon married Himorogi Amane when he was age 13 and she was age 17. While this was her own volition, the Ubuyashiki family has long since married daughters of family closely associated with shrines as any means of trying to use this spiritual influence to lengthen the lives of their offspring. (We may assume Amane served the role of a miko, and she continues to perform daily spiritual ablution to pray for her family’s safety). Of the current Pillars, Kagaya first met Himejima in 1906 at the age of 14. The quintuplets, including Kagaya’s heir Kiriya, would had been born around 1907.
It’s when he’s settled into a husband and (probably step-)father role that he is discovered by a member of the Demon Slayer Corp in 1915, possibly for the first time in roughly four centuries. He makes a clean getaway, but not without those hanafuda earrings freaking him out first.
Muzan is busy being a Tokyo business man (among other things) with interests in foreign language and new contraptions. He is still motivated to discovered all he can about the blue spider lily, which is why he focuses his demons around the Tokyo area. In 1913 he takes a stroll to Mt. Kumotori and snacks on a family out there, tries making one of them into a demon for the heck of it (maybe or maybe not knowing anything about the Kamado lineage), assumes it didn’t work and she died so he forgets about her (giving Nezuko an opportunity to break free of his curse). He finds no blue spider lilies and goes back to Tokyo.
Wanting to get close to business partners, he identifies a high society woman named Rei, kills her husband, and swoops in to be her new lover. At other times, he takes the form of some other power person when it’s convenient, or when he’s juggling multiple identities, like an adopted boy with access to medicinal research or a performer with who can boss humans around to search out the flowers in daytime, he may sometimes leave a piece of his flesh behind to assume his form when he’s busy elsewhere.
And the rest is basically as we see in canon.
Of the Lower Moons, the former Lower Moon 2, Hairou, was a Shinsengumi member, and therefore became a demon sometime between 1863 and 1869. Than means he had been a demon roughly fifty years or less before Rengoku Kyojuro defeated him and became a Pillar.
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