Culling game:
Ritual for kenjaku to trigger assimilation of humanity (or japan i forgor) with tengen (the sorcerer association lied, there was no replacement for Riko Amanai)
About colonies:
Supposedly all non players in the area before the ritual happened are to be evacuated from the boundary if they chose not to participate in the the culling game, like the case with Yuji’s former high school friends, I don’t know what happened in Yuta’s case.
cont- The contract is not with Kenjaku per se but to the concept of “Game Master”, think of it similar to “Heavenly Restriction” By gaining 100 points you can bargain with the “Game Master” to add another rule that doesn’t contradict the previous rules.
About “Idle Transfiguration”:
Immortality does not prevent Tengen’s aging, which causes her to become similar to a cursed spirit or rather a deity. Idle Transfiguration and Uzumaki allows Kenjaku to manipulate and assimilate humanity with tengen(i forgot how and why though).
Anything else?
The Culling Game is basically Kenjaku forcing awakened sorcerers and reincarnated ancient ones into barrier “colonies” across Japan where they have to kill each other to earn points and add rules, all to generate massive, cursed energy and push humanity to evolve.