The Six-Armed Demon belonged to the undying species. Legend says that the undying were a new species that was born from drinking the water at the Fountain of Youth. They were called the undying not because they could not be killed but they had a long life. In this world, most of the human below transcendent had 360 columns on their Life Wheel and each column of Life Light represented one year of life. Even if all of the gray columns were activated at once, the most one could live was less than 720 years. Even gold-level creatures were not exceptions to this unless they upgraded to a transcendent.
Besides humans, most of the monsters below transcendent could not live more than 1,000 years. On the other hand, it was a different story for the undying. If they were not killed, they could easily live up to thousands of years, sometimes 10,000 years. They lived longer than most monsters did.
