A living being that will live forever. Note the technical difference between eternalness and immortality: God is eternal because had He had no beginning. He also is immortal because He will live forever. Humans, angels, demons, etc., are immortal because we will never cease to exist, but not eternal because we did not always exist. It is important to realize that in practice eternal and immortal are often used interchangeably and the technical difference is not always maintained.
To avoid offending anyone here, there are no deities in this LARP. Instead certain PC's have attained immortality, and can grant special abilities to characters that have faith in them, and help carry out their goals.