The last two sentences in the passage, "All I saw was darkness. Oppressive darkness. Darkness that swallowed everything," are examples of rhetorical fragments. Rhetorical fragments are sentence fragments used to emphasize a point.
Rhetorical fragments can be a single word or a long phrase. They are made by cutting out non-essential words in a sentence, allowing the reader to infer. Rhetorical fragments are used for dramatic effect, often to denote confusion or rapid thinking on the part of the speaker. They catch the attention of the reader by disrupting the rhythm of the writing; because of this, they can make the writing sound choppy and are most effective when used sparingly.