Genesis 15:12 - Joseph Benson’s Commentary on the Old and New Testaments

And when the sun was going down About the time of the evening oblation; for, he abode by them, praying and waiting till toward evening; a deep sleep fell upon Abram Not a common sloop through weariness or carelessness, but a divine ecstasy, that, being wholly taken off from things sensible, he might be wholly taken up with the contemplation of things spiritual. 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.