Isaiah speaks of the people living sinful lives and suffering because of it as if they had been walking in a dark or living in a land of shadow of death. (See: [[rc://en/ta/man/jit/figs-metaphor]])
The phrase "shadow of death" is an idiom that means absolute darkness. Alternate translation: "the land of deepest darkness" (See: [[rc://en/ta/man/jit/figs-idiom]])