JJK RATIO LORE
Begining
Since the dawn of man humans have had emotions both positive and negative feelings in their minds. These negative thoughts and emotions have started violence and loss all across time itself and eventually all of this would form a source of energy in the Paleolithic era. This source of energy is known as cursed energy and this energy would form into beings that people would refer to as "demons". Invisible creatures that caused unexplainable phenomenon.
As time went on and people began to be born with larger amounts of cursed energy. A better understanding of these demons would be known to these individuals and they would simply be deemed as "cursed spirits" in the Yayoi era.
Evolution
Combating these cursed spirits was a challenge in the beginning as cursed energy wasn't as refined as it is today. Not many people in the world were able to even see cursed spirits but ones that were able to were known as "sorcerers" or "exorcists". They were tasked with exorcising cursed spirits which tended to end in multiple fatalities just to take on a single spirit as they had abilities and powers that sorcerers at the time didn't have.
But things picked up greatly for humanity in the Asuka era. Gone were the days of sorcerers needing to sacrifice squads of men to combat a single curse as they would become more entuned with cursed energy which would allow them to gain similar abilities to the cursed spirits themselves. These abilities would be the catalyst for early jujutsu. Sorcerers were now able to enhance their physical capabilities with cursed energy and in extremely rare cases even heal others through an extremely hard to learn process.
During the Nara period was when an event called the "discovery" would begin to take place. This would be a period where humans would actually stand on top of curses and sorcerers would force curses into hiding. The reason for this was simple, jujutsu sorcerers had found out how to personalize cursed energy to their will which allowed them to create what is nowadays referred to as a "cursed technique". These cursed techniques would normally come from bloodlines long entrenched in the art of jujutsu sorcery but there were multiple cases were people not in these bloodlines had been born with cursed techniques. In this period there were about 20 main families that ruled in jujutsu society since there were so few casualties from curses and sorcerers were able to actually go and start families.
Things seemed to be in the humans favor and even peaceful for a good portion of the Nara era. But things would change come the...
The Heian era
This was the best and worst period for jujutsu sorcery. There were so many discoveries and advancements in sorcery that it was hard to keep track of. Domain expansions, heavenly restrictions, binding vows, all of that came from this time period and the sorcerers who learned of it didn't have time to wonder around on phones or have many free times. They were left refining their abilities becoming as strong as they could possibly be to kill one man.
Ryomen Sukuna
This mans birth shook the world, not just the world of jujutsu sorcery but the world in its entirety as he was the most evil creature the planet had ever experienced. There wasn't a curse spirit that could compete with him and as he grew in power he gained the title of "the king of curses". He had mastery over everything jujutsu related and with his horrible power he did as he pleased, feasting on those who didn't worship him and eliminating ALL who challenged him.
The Hein era was a time of extreme growth and development but fear at the same time as one could never know when Ryomen Sukuna could make their way in the area and completely remove you off the face of the map. Sukuna invoked so much fear and negative emotion that the sheer mention of his name would create curses far more powerful than before and be the main reason for curses having cursed techniques in the first place. Already existing curses would become so powerful that they'd evolve in strength acquiring similar cursed techniques to that of humans and ones that were created would just be born with them due to how much negativity they'd be created from.
It took all 20 of the main jujutsu families working together to take down Ryomen Sukuna and the army of curses he had created from just existing but it didn't come without cost. A large portion of the jujutsu families were eradicated by
Sukunas hands, reducing the 20 main families down to just 3. Even with their numbers dwindled significantly the families were not able to fully kill Sukuna as he had transformed his body and power into cursed objects that held immeasurable power. Just one of his fingers could grant a curse unimaginable power and if one was able to, they could house Sukuna within their own body. Legends say that if Sukuna were to collect all 20 of his fingers he'd be able to reach the same level of power that destroyed 17 out of 20 of the jujutsu families.
Current Age
In the current age Jujutsu isn't as refined as it was in the Hein era. After the defeat of Sukuna things had died down. There were powerful sorcerers still being born and families making a name for themselves within the other eras such as the rise of the Inumaki family in meiji era. But they never made it to the same standing as the three main families and after the Taisho period things really began slow down. Sorcerers weren't all that obsessed with honing their skills like they were in the old days, instead the most important thing became sheer natural talent. After that era a sorcerers power became 80% cursed technique and 20% everything else and even the curses began to be affected by this shift in ideals and power dynamic.
Jujutsu society wasn't ruled over the main families anymore, they were still a large part of it but the main politics were now done by individuals known as the "higher ups" who simply barked orders at the remaining Jujutsu families. In their primes they were considered tactical masterminds but as they aged their tactical minds began to fade and their decision making became more radical and unethical. Despite this they still remain in power, holding onto what little things they have left in this age as they remain protected by loyalists who hold memories of the higher ups masterful planning.
Gone were the times of where the strong ruled the weak but instead it was the weak ruling over the strong. Now were the days where if a child was born with enough cursed energy they'd be forcefully indoctrinated into jujutsu society by order of the higher ups. The Taisho era saw the rise of both Tokyo and Kyoto Jujutsu schools and Sukuna would become nothing more than a folktale. But the legacy of sukuna didn't end completely as his evil and or selfish deeds coupled with the shaky shadow governments that rules over the main families inspired what would be known today as curse users. Those who do not align themselves with jujutsu sorcerers to fulfil their own goals. Most tended to be evil and some were only curse users by title wishing to live a life beyond jujutsu sorcerery.