Genesis 15:12

And when the sun was going down
Just setting, descending below the hemisphere; or "about to enter" ^F13 into his chamber, as Piscator observes, from whence he went forth in the morning, as a strong man to run his race; which at sunset is finished according to human appearance, and the common apprehensions of men, who have thought it goes under the earth, or drops into the ocean, see ( Psalms 19:5 ) :

a deep sleep fell upon Abram:
through the great fatigue he had had the preceding day, in doing what is before related; or rather through a more than ordinary influence of God upon him, which bound up his senses, and cast him into an ecstasy or trance, when he had the following prophecy and vision, which more fully explained to him the emblem he had been conversant with; this was such a sleep as fell on Adam, ( Genesis 2:21 ) :

and, lo, an horror of great darkness fell upon him;
or such darkness as was horrible and terrible, so it was represented to his mind in vision; which signified the great afflictions after expressed by darkness, that should come upon his children in Egypt and elsewhere: and so Jarchi says it refers to the distresses and darkness of their captivities in Egypt, and in other places. The Targumists observe, that Abram in this vision saw the four monarchies that should bring his children into bondage.