Foreshadowing

A sign, saying, or image that anticipates, sometimes only for the reader who rereads, something that will happen later in the work.

In The Idiot

but I know their secret. [...] I am certain that he has a razor hidden in his drawer, wrapped in silk, like that Moscow murderer [...] I would kill him out of fear... But he will kill me first...

The razor “wrapped in silk” and the phrase “he will kill me first” foreshadow Nastasya's death. What she fears in the letter is what the last part of the book confirms.

Part Three: The Explanation, Nastasya, in a letter to Aglaya

if you start to talk about something like the death penalty, or the economic situation of Russia, or of "beauty will save the world," then... I, of course, will be delighted and laugh heartily, but... I warn you in advance: do not show yourself before my eyes afterward!

Part Four: The Denouement, Aglaya