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So, I've seen a lot of comments of how confused you guys are and I am here to give you a (hopefully) understandable explanation of what is going on.
At the end of the Shibuya-Arc, Kenjaku (the brain/Fake-Geto) used Mahito's Idle transfiguration on two types of Jujutsu sorcerers to adjust their brains for sorcery: People who ingested Cursed Objects (like Yuji) got strength as vessels and people with Cursed Techniques whose brains were meant to be Non-sorcerers (like Junpei) got the ability to use Cursed Techniques. Adding to that he broke the seal on Cursed Objects, some of them already over a thousand years old (Kurusu Hana, the angel-person, is one of them if I'm not mistaken, because they are over a thousand years old). Kenjaku is now planning on making these three types of people fight each other in the culling game, to deepen their understanding of cursed energy and finally merge them with Tengen to create a new kind of jujutsu sorcerer, an evolved human.
Now on to the culling game, which is probably the most confusing part for you guys: Kenjaku (and most likely other sorcerers/curse users form the old times) created 10 colonies all over Japan. These colonies are used as a preparation to turn the people in Japan into curses to merge with Tengen (I'll address this point later).
There are 8 rules in the culling game, all dependent on each other. I'm going to list all of them in one go, because I think it's easier to make sense of them if there isn't text in between them.
1. After awakening a cursed technique, players must declare their participation in the culling game at a colony of their choice within 19 days.
2. Any swimmer who breaks the previous rule shall be subject to curse technique removal (this is just a nice way of saying to kill the swimmer)
3. Non-players who enter the colonies become players at the moment of entry and shall be considered to have declared participation in the Culling Game.
4. Players score points by ending the life of other players.
5. Points are determined by the game master and indicate the value of a player's life. In principle (as a general rule), sorcerers are worth 5 points and non-sorcerers are worth 1 point.
6. Excluding the point value of a player's own life, players may expend 100 points to negotiate with the game master to add one new rule to the culling game.
7. In accordance with the previous rule, the game master must accept any proposed new rule unless it has a marked and long-lasting effect on the culling game.
8. If a player's score remains the same for 19 days, that player shall be subject to cursed technique removal.
So in short, Itadori, Megumi, Yuta, Maki and Panda (I don't know about the others, and Nobara's status is still unknown - Whyyyyy Gege, Whyyyy??) will have to join the culling game and score 100 points so they can add a rule to the already existing rules. The reason is because they want to save Tsumiki, Megumis sister.
Another reason why they must join the culling game is to get in contact with Kurusu Hana, the only person who is capable of terminating all techniques. If Yuji & Co. manage to get them on their side, Kurusu can terminate the technique keeping Gojo inside the cube (Though we don't know who this Kurusu Hana person is and on whose side they are. We'll see in the next chapters how it'll turn out)
We are still going to see what kind of solution Megumi has so they can earn points by not killing people, but from what has happened so far in the manga, I am pretty sure that somebody has to kill or will be killed.
Now what happens if humans are merged with Tengen. First the basics: Because there isn't enough cursed energy to use Idle Transfiguration on every Japanese person to transform them into a sorcerer, Kenjaku is trying to achieve the wanted evolution by merging the humans with Kenjaku. By merging them, the humans surpass the normal definition of a sorcerer. They exist but also don't exist, which leads to a new form of existence. These evolved humans lack the boundary of an individual and if one of those evolved humans go out of control it'll be the end of the world / the Shibuya-Incident will repeat itself all over the world.
Another huge problem is the fact that Tengen is susceptible to curse spirit manipulation, which means that Kenjaku will absorb him immediately upon contact. This is the reason why Tengen wants/needs guards (Yuki and Choso in this case). Everything will be over for the sorcerers if Tengen and his barrier disappear.
I also recommend binge reading from chapter 135 onward until the current chapter. It'll help a lot
Hope this was to some point understandable and you get the story a little better ヾ(❀╹◡╹)ﾉ~
I am open for any improvements and constructive criticism as well as corrections in case I got something wrong.