Tengen’s Immortality, first talked about in the Hidden Inventory arc, might sound like a near perfect cursed technique at first. But the more we get to know about its details, the more of a hassle it seems.
First off, the only good part about the technique is that it grants the user immortality. However, it doesn’t stop the aging process. Sayonara eternal days of youth!!
But that’s not even the biggest problem here. Master Tengen’s immortality technique has a very specific condition. After a certain age limit has reached, the cursed technique will cause a forced evolution of the body.
In order to stop this, Tengen has to merge with a compatible star plasma vessel every 500 years. This subsequently resets the cursed technique from going awry. But then, evolution is something that’s usually supposed to be good right? So why are the sorcerers wary of it? What happens if Tengen doesn’t merge? What happens if they evolve?
Well to answer in short, if Tengen evolves, they will ascend to a higher state and lose their sense of self. If that happens they might even turn into an enemy of humanity.
To understand why this would happen, we need to delve deeper into the consequences of Tengen losing their sense of self.
Note: If you are curious about Tengen’s evolution as a part of Kenjaku’s plan, click on the link below!
Read More: Why did Kenjaku start the Culling Game?
Tengen’s evolution and losing their sense of self:
When discussing Tengen’s evolution, a crucial point to consider is the gradual disappearance of their will. Suguru mentions this while explaining the concept of evolution to Satoru in chapter 66.

Similarly, in chapter 145, Tengen reveals that as their aging accelerates, their individual self-awareness diminishes.

From a psychoanalytical perspective, both statements point to the phenomenon known as ego death – the fading away of one’s sense of self. Interestingly, ego death is also recognized in many religious and shamanic practices as a means to achieve enlightenment and forge a connection with the universe and the divine.
It is possible that Gege Akutami drew inspiration from this concept for Tengen’s evolution. This leads me to infer that Tengen’s evolution is accompanied by the dissolution of their ego.
But why is this significant?
To comprehend its importance, we must first grasp the role of the ego and its importance in maintaining one’s sense of self.
We are entering Freud-ulent territories here.
According to Freud’s theories, the ego represents the conscious and self-aware aspect of an individual’s personality. It acts as a mediator between the unconscious id, which embodies primitive and instinctive drives, and the superego, also unconscious, which enforces moral standards.
The ego’s function is to find a realistic balance between the demands of the id and the constraints imposed by societal norms. In essence, it keeps us in check, regulating impulses and impetuous behavior.
When a person experiences ego death, their ability to make conscious and rational choices diminishes. Moreover, it grants the id unchecked control, akin to a wild horse without a rider. Depending on its course, this can lead to disastrous consequences.
The id consists of two primary biological instincts: eros and thanatos. Eros represents the life instinct, encompassing desires for love, pleasure, and connection. It drives us to seek happiness. On the other hand, thanatos symbolizes the death instinct, manifesting as aggression, destruction, and even self-destructive tendencies.
If Tengen evolves and loses their ego, there is a risk that they may gravitate towards the thanatos persona, hence Geto’s suggestion that they could potentially become an enemy of humanity.

However, the reason why this possibility is particularly catastrophic extends beyond our initial imagination.
Tengen’s barriers – The cornerstone of Jujutsu in Japan:
It might not seem so at first, but Tengen is one of the cornerstones on which the Jujutsu society in Japan is built on. As Geto mentions, all barriers which act as the pillars of the Jujutsu world are strengthened by Tengen.
Additionally, it is because of Tengen’s barriers that it is possible to optimize cursed energy in Japan, which in turn lets Japanese people become sorcerers. Because, the fundamental part of becoming a sorcerer is the optimization of cursed energy.
As pointed out by Kenjaku in chapter 220, Tengen’s pure barriers help suppress the cursed spirits and also strengthen the barrier techniques of the assistant managers. If anything were to happen to these barriers, the fight with cursed spirits would regress.

To add to it, the sorcerer’s knowledge about the barrier techniques would go back to what it was a thousand years ago. This would make it hard to even take up missions or put up curtains and other basic stuff.
So, if Tengen were to evolve and lose their self as I mentioned above, and end up embracing their Thanatos persona, then not only would the barrier techniques be lost, but the conditions that are applied on pure barriers, which currently optimize cursed energy and suppress cursed spirits, might reverse and bring forth something more sinister.
If such a scenario comes to pass, then Tengen would surely become an enemy to humanity as Geto aptly stated.
I hope this clears up why it is important for Tengen to merge with Star Plasma Vessel. Sure, most of it is me just going into the nitty gritties of what could happen, but it still paints a gruesome picture.
If you still have any doubts regarding this, feel free to drop a comment below, or hit me up on Tumblr/Reddit!