As a student of immortality magic and all it entails, I have come across what seems to be two very distinct kinds of immortality.

Many creatures can claim to be immortal for one reason or another, but in the end it all boils down to whether or not you can die. I like to call these two types of immortality "conditional" and "unconditional."

With "conditional" immortality a person or creature may enjoy the benefits of an extended lifespan with the caveat that they can die under the right circumstances.

Vampires claim to be immortal and while there have been no records of a vampire dying of natural causes, there are certainly what I like to think of as "generations" of vampires. The oldest generation seems to average between 400 and 600 years old before what I call 'the law of stupidity'* sets in.

A lobster is theoretically immortal as long as it has enough energy to shed its shell infinitely, lest it die of being crushed to death by its own terrible exoskeleton, but plenty are killed and eaten every day by average humans.

Other examples of conditional immortality include humans who have been given extended life in exchange for never breaking a certain rule set at the time that immortality was granted, meaning that they can theoretically be killed if the rule is broken; and the humble tardigrade, which apparently explodes when shot from a gun.

"Unconditional" immortality on the other hand serves to not only extend the lifespan of a creature, but also etches its form into the very fabric of the universe like an ugly scar.

A creature with unconditional immortality may be harmed to any extent and will return to its original form given enough time. They are essentially a fact of life.

Regenerative capabilities are essential to the very notion of unconditional immortality, as without such it is just a variation on conditional immortality.

The only creatures which I have observed to exhibit unconditional immortality are certain elder gods - creatures which have no natural predator besides each other. As much as humans love to dream of killing their gods, it is laughable to even propose that humans are the natural predators of gods. It is far more common for a god to be sealed in a dormant state than to truly die.

It is said that the elder gods will die when the universe comes to a close - perhaps their lifespan is tied with that of the universe itself.

*The law of stupidity dictates that the longer you live, the higher the chance of doing something possibly life ending. While there appears to be no record of a vampire dying of old age, there are plenty of records of vampires doing something entirely preventable and stupid.