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

Wesley's Notes

15:12 And when the sun was going down - About the time of the evening oblation. Early in the morning, while the stars were yet to be seen, God had given him orders concerning the sacrifices, Ge 15:5, and we may suppose it was at least his morning's work to prepare them, and set them in order; which when he had done, he abode by them praying and waiting 'till towards evening. A deep sleep fell upon Abram - Not a common sleep through weariness or carelessness, but a divine extasy, that being wholly taken off from things sensible, he might be wholly taken up with the contemplation of things spiritual. The doors of the body were locked up, that the soul might be private and retired, and might act the more freely. And lo, a horror of great darkness fell upon him - This was designed to strike an awe upon the spirit of Abram, and to possess him with a holy reverence. Holy fear prepares the soul for holy joy; God humbles first, and then lifts up.

Matthew Henry's Concise Commentary

15:12-16 A deep sleep fell upon Abram; with this sleep a horror of great darkness fell upon him: a sudden change. The children of light do not always walk in the light. Several things were then foretold. 1. The suffering state of Abram's seed for a long time. They shall be strangers. The heirs of heaven are strangers on earth. They shall be servants; but Canaanites serve under a curse, the Hebrews under a blessing. They shall be suffers. Those that are blessed and beloved of God, are often sorely afflicted by wicked men. 2. The judgment of the enemies of Abram's seed. Though God may allow persecutors and oppressors to trample upon his people a great while, he will certainly reckon with them at last. 3. That great event, the deliverance of Abram's seed out of Egypt, is here foretold. 4. Their happy settlement in Canaan. They shall come hither again. The measure of sin fills gradually. Some people's measure of sin fills slowly. The knowledge of future events would seldom add to our comfort. In the most favoured families, and most happy lives, there are so many afflictions, that it is merciful in God to conceal what will befall us and ours.