In Part 3 of the story, the narrator shifts their focus to a Union officer who has wandered out into the open of the valley and witnesses the horseman’s fall from the cliff after being shot. In this moment, Bierce includes a subtle allusion to the Bible, as seen in the following passage:
Filled with amazement and terror by this apparition of a horseman in the sky — half believing himself the chosen scribe of some new Apocalypse, the officer was overcome by the intensity of his emotions; his legs failed him and he fell.
When Bierce describes how the officer “half believ[ed] himself the chosen scribe of some new Apocalypse” after witnessing the “apparition of a horseman in the sky,” he is alluding to the Four Horsemen of the Apocalypse, as described in the Book of Revelation in the New Testament. While many consider the four horsemen to be personifications of death, famine, war, and conquest, others interpret them as literal horsemen who may appear as harbingers of the end of the world. Here, the wandering officer sees the horseman in flight and, likely unconsciously, makes a connection between the biblical imagery and the sight in front of him.
Bierce’s inclusion of allusions like this adds to the haunting, horror-esque nature of his writing. While this story is about the relationship between Druse and his father, it also about the horrific nature of war generally. In other words, it makes sense that the wandering officer would consider this horseman a sign of the apocalypse, as he is living through the rather apocalyptic experience of the Civil War, full of terror, violence, and a large number of causalities on both sides.