Matthew Poole and continuators on Genesis 15:12

12 And when the Sun was going down, a deep sleep fell upon Abram and lo, an horror of great darkness fell upon him .

Partly natural from his labour in killing and sacrificing those creatures: and partly sent upon him from God, to make way for the following representation.

(i. e.) He seemed to be covered with a dreadful darkness, which was, either 1. A Token of God's special presence: compare 1 King. 8. 12. Or, 2. A signification of the distressed and doleful condition of Abram's seed: for darkness in Scripture is frequently mentioned as an emblem or sign of great misery, as Psal. 88. 6. and 107. 14, &c.