Jujutsu Kaisen's main story centers around Yuji Itadori consuming one of Ryomen Sukuna's fingers and the King of Curses awakening within his body. This journey begins when a Curse starts attacking the protagonist's school. However, many people often forget that the series never explains the origin of this Curse.
It has been established that Curses in Jujutsu Kaisen are formed from the accumulation of negative emotions due to a specific element or cause. However, the story does not clarify the origin of this being, though there are various theories, including events that transpired in Yuji's school or Kenjaku planting the seeds for this clash.
Disclaimer: This article contains spoilers for the series and reflects the author's opinions.
Explaining the possible origins of the Curse that attacked Yuji's school at the beginning of Jujutsu Kaisen

As far as the actual story written by author Gege Akutami is concerned, it is never explained where the Curse that attacked Yuji Itadori's school came from. Therefore, there are a lot of theories to go by, such as Kenjaku planting this creature to ensure Yuji would consume Ryomen Sukuna's finger and kick-start the events of the story, which aligns with his nature as a planner and manipulator.
Another theory suggests that this being emerged from years of rumors about a dead body buried in that school. This makes sense, considering that Curses are formed from accumulated negative energy combined with the negative emotions that students may have experienced over various topics throughout the years.
It is also worth noting that the school's sports team fell ill due to tick bites, and this Curse resembles a tick, as demonstrated during its fight with Yuji Itadori and Megumi Fushiguro. This doesn't mean that the Curse itself was biting the students, but it may have arisen from these negative emotions.
The issues with the different theories

It is very likely that Gege Akutami did not have a planned origin for this Curse in Jujutsu Kaisen and intended it only as an obstacle for Yuji to consume the Sukuna finger and become the latter's vessel. However, it makes sense for Kenjaku to be responsible for this, as he orchestrated the protagonist's birth to make him the ideal vessel for Sukuna; thus, it would be logical for him to have created this situation.
It is quite difficult to determine the origin of this Curse because, as mentioned earlier, there is a strong chance that this creature was conceived merely to generate a scenario for Yuji to begin his story. Therefore, not much can be gleaned from this monster's role in the series beyond the fact that it was a necessary step to propel the story forward.
Final thoughts
There is no information about the origin of this Curse in Jujutsu Kaisen, and it was mostly used to get the story going, so it is likely that Akutami never intended for that aspect of this creature to be developed. However, it is fun to theorize about how this Curse might have come into existence.
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