Immortality

Immortality is the state of having possess an infinite lifespan. It affords the user immunity to the effects of aging and diseases, thereby granting the ability to outlive other supernatural creatures and humans. Some beings (i.e. Deities, Demons, Angels, Ghosts) are immortal by their nature. The ability applies only to those once mortal.

Use
The ability may be bestowed upon a mortal by a powerful being (i.e. Deities, Demons, and Spirits) though that usually requires a dubious deal involving souls or sacrifice, one might also be afflicted by a curse.