Immortality

"Immortality" is a technique created by the mythical Haji Valtchy thousands of years ago. According to the Senshist legend, he discovered while trying to create a technique with the power to eradicate the universe. The technique changes the way the user's cells work, and makes it so that the user will stop aging immediately, thus giving them eternal youth, and potentially, eternal life.

Haji Valtchy
Haji Valtchy created the technique, and used it to obtain several lifetimes worth of training and research, which allowed him to learn all the Elements and Techniques in the universe at the time. He eventually developed the Truth: Death technique, and used it to annihilate entire galaxies, and to take control of the entire universe with the assistance of the Valtchies.

Senshi
According to The Senshist Book,Senshi awakened the technique after the loss of his best friend and lover at the hands of his rival. Senshi used to the technique to learn all the secrets of the universe and its species, and at the same time, he grew stronger with experience. Wishing to end The Valtchy Regime, he challenged Haji to a duel. After defeating and banising Haji from their dimension, Senshi's immortality deactivated, and he spent the rest of his life preaching about peace and love.

Murasaki
Murasaki awakened the technique on her deathbed, and she used it to transfer her life force into The Wind Statue. All of her followers obtained a fraction of her powers thanks to the statue's influence.

The Chickenwing Commanders
Two of the Chickenwing Army commanders during The Koji War discovered a book containing the technique, and used it for their own benefit. The legendary Rokun used it on himself, and then vanished. Mayor Mu used it to rule Onion for over 300 years. For unknown reasons, Toshiro Chickenwing didn't use the technique on himself, but instead he used it on his son, Brad Chickenwing, and died shortly after the book's discovery.

The Masked Man
Yashonaru claims to be a veteran of the Koji war and a former friend of Kosho's, making him around 400 years old, but he has a young body and voice, hence why Rokko Chickenwing stated that he's most likely under the technique's influence.