What category does the word prologue belong to? Is it a sound pattern, a literary device, a type of narrative, or a part of a narrative?
It is a part of narrative.
It is a part of a story (book, film etc.) set before the main events in the story.
For example there is a prologue in the movie "The Lord of the Rings: Fellowship of the Ring"
This part of the film introduces how the rings were made and what other events led to the main story.