Master Tengen is one of the most mysterious and important characters in Jujutsu Kaisen. They are the sorcerer that keeps up the protective barriers around both Jujutsu High locations and are the founder of modern jujutsu -- the entire jujutsu system is based on their teachings. However, Master Tengen also has the potential to completely destroy the jujutsu world and humanity.
Over 1200 years ago, during the Nara Period, Tengen was a Buddhist monk who traveled, preaching a belief that would become the precursor to the current jujutsu system. Tengen is immortal, but not eternally youthful, and so Tengen's body wears down and ages. If they don't find a new vessel every 500 years, Tengen's body will try to force itself to find a new vessel. The technique takes on a life of its own if a new vessel is not merged with within that time frame.
11 years prior to the current storyline, in 2006, Tengen was set to merge with a young girl named Riko Amanai, but she was killed by the Time Vessel Association before the merge could take place. She was a middle school student whose parents had died in a car accident and was being looked after by Misato Kuroi until the time of the merging. Because the merge never happened, Tengen's physical appearance began to evolve, so now they have four eyes, no hair, and a cylindrical head. Most people do not see them as they hide, for their own safety.
If a new vessel is not found in time, Tengen will undergo a forced evolution and eventually lose their sense of self, making them highly dangerous to all humanity. Two groups have attempted to stop the merges, one being the religious Time Vessel Association, who view Tengen as a deity and pure, making merging with a Star Plasma Vessel like Riko sacrilegious. They have officially disbanded since Riko's assassination. The other is a group of curse users called Q. Made up of only two members, the group wanted to force the evolution of Tengen so that they would potentially become an enemy to the jujutsu schools and humans. They have also since disbanded. While these two groups no longer exist, that doesn't mean Tengen isn't still a target.
Because of the dangerous nature of Tengen's abilities, it's important for them to stay hidden until it is time to reset the immortality curse technique. Their barrier technique is what keeps the schools safe and is what changes the schools' appearance every day. However, if someone infiltrates the barriers, they become more than useless, making it important that Tengen's location remain a secret. The barrier protecting them is one that only allows those who are invited in to pass through.
Because they were unable to merge with Riko, Tengen is now more cursed spirit than human, making the possibility of merging with someone who isn't a Star Plasma Vessel more likely. However, this isn't good news because it makes them more susceptible to manipulation, particularly by Kenjaku, who is looking to force human evolution in Japan. If Kenjaku is able to merge with the vulnerable Tengen, the negative feelings and intentions of one person would spread to everyone because once a human evolves, they lose their sense of identity and self. Humanity would become a collective hive-mind that would follow the most powerful thought, that being the negative impulses of Kenjaku. This is what would lead to the end of the world.