As implied by its name, Immortality grants the user the ability to prolong their life, effectively granting them the potential for an infinite lifespan. However, this technique is not "de-aging", meaning that the user cannot escape the physical changes brought upon by the process of aging.
Star Plasma Vessel
Once a user of immortality reaches a certain age, the technique attempts to change bodies by undergoing an "evolution". This process causes the self-awareness and will to exist to cease and the user becomes a shell of their former self as a result. Every five hundred years or so the user must merge with a capable Star Plasma Vessel (
Tengen has existed since the Nara Period and laid the foundations of what would become the culture surrounding jujutsu sorcery. They have become known as Master Tengen, an Immortal hermit who does not interfere with the world. Connected by fate to the Six Eyes and the Star Plasma Vessel, about every five hundred years Tegen must merge with a compatible vessel. The most recent occurrence was in 2008, eleven years prior to the current day.
Satoru Gojo, the current wielder of the Six Eyes was tasked with escorting the most recent Star Plasma Vessel, Riko Amanai. Jujutsu High feared that Master Tengen's evolution would compromise the barriers protecting the schools, and in the worst-case scenario, cause Tengen to become an enemy to humanity.
The Time Vessel Association is a Star Religious group that worships the immortal Master Tengen. They believe merging with the Star Plasma Vessel would be impure and hired Toji Fushiguro to assassinate Riko. Toji was successful and prevented the merger. It was initially believed that Tengen stabilized because another vessel may or may not have appeared, but this was not actually the case.
After failing to merge with a Star Plasma Vessel, Tengen's aging accelerated, and their self-awareness diminished, causing them to become "one" with the world. They evolved for the past eleven years and now it is possible for someone other than a Star Plasma Vessel to merge with them. Tengen has become an evolved soul capable of merging with a human and making them something beyond a jujutsu sorcerer. This evolution has caused Tengen to become more cursed spirit than human, making him a prime target for Kenjaku's plan. 
- Jujutsu Kaisen Manga: Chapter 53 (p. 5).
- Jujutsu Kaisen Manga: Chapter 66 (p. 3-5).
- Jujutsu Kaisen Manga: Chapter 145 (p. 9).
- Jujutsu Kaisen Manga: Chapter 66 (p. 3-6).
- Jujutsu Kaisen Manga: Chapter 66 (pp. 8, 18-19).
- Jujutsu Kaisen Manga: Chapter 74 (p. 4-5).
- Jujutsu Kaisen Manga: Chapter 77 (p. 14).
- Jujutsu Kaisen Manga: Chapter 145 (p. 2-7).