Definition
A very dangerous place.
Origins
From Psalm 23 of the Book of Psalms. "Shadow of death" is a calque of Hebrew צלמות.
How People Actually Use It
The Valley of the Shadow of Death is a place where soldiers fight to the death and are protected by god above. God Save The Marines Semper Fidelis
"Today, we enter the Valley of the shadow of death. Men will die. Dead or Alive, We will leave no-one behind. I will be the first to step on the field and the last to step off."
Source: Urban Dictionary