Immortality is a nebulous umbrella term used to describe any being that can potentially live forever. There are several different degrees of immortality all with their own caveats.
The curse inflicted upon the mortal shell keeps it frozen in time, but requires an appropriate blood sacrifice to rewind the clock when injured. Undead can exist indefinitely unless their heart, or a significant amount of their body is destroyed and drained of the blood necessary to repair. Since these beings are in a constant state of death it is not considered true healing when tissue is regenerated because that would mean new cells are being given life. Instead, it is the same cells that are brought back by the curse time and time again.
When an undead dies or part of the body is removed from its connection to the heart, the curse is considered to be void. Depending on the age and damage sustained throughout their existence under the curse, the undead body may appear as a decaying corpse if young. Older individuals are reduced to ash from years of physical harm and cells that should have expired naturally long ago.
A living body in transcendent harmony with itself can exist infinitely. The body continues all life processes, but can still die upon sustaining lethal damage. Accelerated healing is not a function of this immortality and should be considered an entirely separate trait. There are four levels of this condition.
i. The body continues to age normally. Eating, sleeping, and breathing are still required.
ii. The body stops aging. Eating, sleeping, and breathing are still required to a lesser degree.
iii. The body stops aging and eating, sleeping, breathing are no longer required. Discomfort and catatonia may occur if the body is deprived for long periods.
iv. The body stops aging and eating, sleeping, breathing are no longer required. There are no negative effects from deprivation if energy is gained through another source.
All constructs of aether—from wisps to elementals to gods—are considered to be undying regardless of strength because they are a manifestation of pure, self-sustaining energy. Aether can be reformatted, dispersed, and crystallized, but never destroyed. Spirits will always return in some capacity if the aether is influenced in the correct way.
These beings supersede existence itself. The concepts of life and death mean nothing to them as they have been around before time began and still will be after it ends. They are aspects of the universe beyond human and most immortal’s comprehension.