Unlike the gods who are both ageless and immortal, Eternals age (albeit at a significantly slower rate than mortals) and can be killed if enough damage is dealt to their physical form. Eternality is not necessarily passed by blood, though having two Eternal parents essentially guarantees it and having one or more Eternal parents does make becoming a one easier. A familial line of Eternals may stretch back to the birth of Saga itself.

Becoming Eternal is not exclusive to birth alone. Eternals can be born at random in any sapient Digimon, with no outward signs of their latent potential. There is an incredibly remote chance that any given sentient, natural being will be born with a Eternal's spark. When that being is put through a period of extreme stress -- in many cases death -- the spark can trigger, causing the individual to ascend and become a Eternal. Once ascended, an Eternal can physically manifest the full measure of their power and Eternal essence, becoming godlike themselves in their capabilities.

No one is quite sure how the Eternals came to be in Saga. Even the gods remain tight-lipped about the occurrences in the ancient past that saw the Eternals' arrival. Some believe that the Eternals are a line of gods themselves, a type of "New God" with simply less-refined qualities when compared to the "old gods" of the Council. Others claim that the Eternals are descended from the gods, evolved demi-gods meant to replace the old. Still others believe that the Eternals are the next step in mortals transcending their limitations and becoming gods themselves. Whatever the truth, the uncertainty and threat of the Eternals once spurred the gods into a campaign that engulfed the entire world in a war in their attempt to exterminate the Eternals. During this time many Eternals found refuge in the faraway continent of Asgardia under the protection of Bor Burison, who was himself an Eternal from a rather noteworthy line.

It was only the emergence of a shared enemy that caused the two sides to put aside their animosity and join together. A tenuous peace has held since then, with Eternals no longer threatened by the gods with annihilation.