As the sun was setting, Abram fell into a deep sleep, and a thick and dreadful darkness came over him.
Genesis 15:12 (NIV)
The rabbis say that sleep is a microcosm of death. How would Adam have known to fear death until he was terrorized with unnatural sleep?
And here in Genesis 15, we see it plainly. Darkness hasn’t been mentioned since Genesis 1. This is an echo of Genesis 2.