undead; immortal being
Colloquial term used in fantasy, gaming, and light novels for beings that cannot die or are already dead but still animate. Not a formal academic term.
In that game, you need a holy weapon to defeat the undead.
彼は不死者の呪いを受けて永遠に生きることになった。
He was cursed by an immortal being and ended up living forever.
Compound of 不死 (ふし, 'immortality') + 者 (しゃ, 'person'). The word is a modern coinage popularized by fantasy and gaming culture.