noun
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.
そのゲームでは、不死者を倒すには聖なる武器が必要だ。
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.
不死身 means 'invulnerable' or 'immortal body', often used for someone who cannot be killed. 不死者 refers specifically to an undead being or immortal person, not the quality itself.
Compound of 不死 (ふし, 'immortality') + 者 (しゃ, 'person'). The word is a modern coinage popularized by fantasy and gaming culture.