Charles Ellicott Commentary Genesis 15:12

"And when the sun was going down, a deep sleep fell upon Abram; and, lo, a horror of great darkness fell upon him." — Genesis 15:12 (ASV)

When the sun was going down. —The time described was the evening following the night on which he had received the assurance that his seed should be countless as the stars. He had then, in his trance, also asked for some security that Canaan should be the heritage of his posterity, and in answer had received the command to arrange, on a large scale, the ceremonial of a solemn treaty-making.

The morning had been spent in the performance of the command, and afterwards he had watched, probably for several hours, by the side of the divided bodies, uncertain what would happen, but occupied in driving away the vultures, which gathered from all quarters around the abundant feast. At sunset the revelation came to him, not in a waking trance, as on the previous night, but in a deep sleep, and with those accompaniments of terror so powerfully described in Job 4:12–16, and which the creature cannot help but feel when brought near to the manifest presence of the Creator (Daniel 10:8).

Lo, an horror of great darkness fell upon him. —Heb., lo, a terror, even great darkness, falling upon him. The terror was not mental so much as bodily, caused by a deep gloom settling around him, such as would be the effect of an eclipse of the setting sun, and shutting all mortal things away from his view.