An immortal Jujutsu Sorcerer and Buddhist preacher from the Nara Period, the historical period that proceeded Heian which was the Golden Age of Jujutsu, Tengen has yet to make a physical appearance in either the manga or anime but his influence can be felt everywhere.
For starters Tengen is ultimately the reason that Geto turned against humanity, having been repulsed by the behavior of the people he was sworn to protect during the incident with the Star Plasma Vessel.
What is a Star Plasma Vessel you ask? Good question. We don’t quite know and I don’t fully trust the information that we do have.
Supposedly Tengen’s immortality depends on being able to merge with another individual born every five hundred years or so, if he were to forgo this process then his Cursed Technique would force a change in his body and consciousness, evolving him to a higher state of being but also robbing him of his current will and perhaps making him a threat to humanity. Merging with the vessel rewrites his aging body’s genetic information and resets his Cursed Technique.
But no, you read that right, the Buddhist monk is parasitizing children because he’s literally afraid to reincarnate.
I take two major issues with the explanations of this process as given.
If Tengen has delayed this process indefinitely how are we even able to speculate on what might happen? Best guess, the same things happened to whoever had possessed the technique before Tengen. That’s almost a given, but the rest of this is just theory crafting. Perhaps in order to spread Buddism in Japan he had travelled to either India or China where this had already happened to someone else from whom he inherited his Cursed Technique by whatever means. So when he brought back Buddhist sutras he also brought the art of Jujutsu? That’s hardcore speculation though.
It is possible that Kamo Noritoshi (no, the other one) was an incarnation of this theoretical previous Star Plasma Entity since we know him to be at least 150 years old but he makes reference to having been active at least a millennium ago. That would still make him younger than Tengen but it’s possible he’s even older than what he’s given away so far. This would give us a valid precedent for Tengen’s misgivings concerning his evolution. Unless the translation is dodgy, in which case it’s possible he said something like “age” “era” or “epoch” rather than millennia, though Noritoshi as pictured already had the distinctive forehead stitching that indicated the possession Cursed Technique it’s worth noting. Perhaps the Cursed Technique forces the user to become an actual Curse themselves? “Geto” seemed to imply that he felt there was no particular difference between humans, Curses and Jujutsu Sorcerers beyond chance making of them what it would and if he has already undergone this kind of apotheosis it would make sense from his point of view.
We do actually have a prior case for this as well in the form of the Prison Realm, the last remains of another monk (a third Plasma Vessel perhaps?) which became or was made into a prison capable of containing anything. We know that powerful Curse Users and Jujutsu Sorcerers can be made into Cursed Objects after they die as this was the plan for Yuji’s body but I dunno. Why would a monk become a prison? I don’t get it unless this was something unintended. It doesn’t really look like other tools such as Playful Cloud or Megumi’s black dao sabres, in fact the inside of the gate looks like flesh and you could argue therefore that rather than having been made into this it’s possible Genshin evolved into this for whatever reason.
Secondly if the Star Plasma Vessel is necessary to stabilize Tengen’s evolution then why is he still alive? My working assumption is that the Star Plasma Vessel is actually the rightful inheritor of Tengen’s Cursed Technique whom he has absorbed or otherwise eliminated numerous times across his long life. I think the ritual merging and the Star Religious Association are two arms of the same system of control and with the help of Toji it just so happens that the latter won out in this case.
Keep in mind also that Gojo and Geto were told that delivering the vessel to Tengen was the same as erasing her. Riko seemed to believe that her merger with Tengen was on equal terms and that her will would persist after her physical body was integrated into his, but nothing else we’re told seems to indicate this.
On another note Tsukumo Yuki expressed to “Geto” that to optimize Cursed Energy Tengen’s barrier was necessary and that a disproportionate number of Cursed Spirits appear within the isles of Japan as compared to the rest of the world. I think when Tengen inherited his technique the barrier that he created somehow centralized Cursed Spirit coalescence within Japan. This makes sense since an event of this kind and magnitude would lead naturally into the Golden Age of Jujutsu, which indeed came to pass in the Heian Period which followed Nara.
In this scenario it’s possible that Sukuna was considered a villain because, aside from being a murderous psychopath, he went against Master Tengen who had monopolized the power of Jujutsu along with the big three Sorcerer families, a system of control that would eventually be embodied by the council of Higher Ups whom Gojo refers to on several occasions. Though I think some fans stretch things to attempt to rehabilitate his character, I have no particular issue with imagining Sukuna to have been the Gojo of his own era and it is worth noting that the legendary Ryoumen Sukuna is considered both a demon and a hero depending on different regional mythologies.
So yes, my running theory is that the darkest thing in Jujutsu Kaisen is an immortal parasite that hoards the power of Jujutsu to itself and a few co-conspirator families, who made the isles of Japan the capital of Cursed Spirit formation, founded the Golden Age of Jujutsu and is the ultimate author of every Curse related tragedy every since.