For the Eternal Life Realm...
What was the most fundamental essence of this realm?
The answer: eternal life.
Just as the names of the preceding realms had reflected their fundamental nature...
The Eternal Life Realm, a realm whose cultivators could live forever, was no exception.
Once cultivators of the Main Spacetime reached this realm, they automatically gained its fundamental traits. They transcended worldly limitations; even the mighty force of time could not easily wear away their lives.
Take their bodies, for example.
Under the effect of those traits...
The cells within a cultivator's body automatically gained the quality of eternal life.
Unless an external force destroyed them or the cultivator had some need to change them...
Their cells would no longer age like those of mortal beings from the moment they advanced. Instead, the cells could absorb various forms of energy to sustain their vitality and health, keeping them in peak condition.
Likewise...
The same held true for their souls.
Unless time was imbued with some special power, or some other exceptional circumstance arose, the souls of those in the Eternal Life Realm could remain at their peak forever, even as time continued to blow across them. They suffered no natural wear, had no limit to their lifespan, and would always remain at their peak.
This was why the Eternal Life Realm was widely regarded as a realm of particular significance.
Often...
When people in the Main Spacetime spoke of gods, immortals, and demons, they were referring to beings in the Eternal Life Realm or above.
In the end...
An unlimited lifespan was among the most basic and powerful desires of all living things.
From intelligent beings with independent personalities to mindless insects, all were the same.
Once you gained that trait and power to match, everyone would accept that