- (idiomatic) A symbolic description of the world, meaning darkness and death are (symbolic) valleys on earth one must walk through, that is, part of the human experience.
- Yea, though I walk through the valley of the shadow of death, I will fear no evil: for thou art with me; thy rod and thy staff they comfort me... - Psalm 23.
- (idiomatic) A very dangerous place.
From Psalm 23 of the Book of Psalms. "Shadow of death" is from the Hebrew צלמות (tzal "shadow" + mavet "death").