TL;DR: A prologue isn’t always necessary for a hero’s tale, but if included, should come out of necessity, less it slows down the narration. It should also attempt to encapsulate the essence of the story.
The story of a new hero should start with the seeds of the old, beginning and ending, death and rebirth. Therefore there might be a story before the story, a prologue, which, as all good prologues do, contain the essence of the narrative to come so the audience knows what’s in store. We see this in many massive fantasy and sci-fi epics, in many cases disengaging the readers right from the very beginning. Whatever the case, the conflicting ideas of the story are in many cases represented by the old hero and his foe under the black and white, almost indistinguishable dichotomy of good and evil.
Although there are many examples, the best I can think of is the confrontation between Lews Therin Telamon and Elan Morin Tedronai in the prologue of Robert Jordan’s The Eye of the World.
If the essence of the book is supposed to be battle centered then it is more likely for the representations of good and evil to fight. Considering so many fantasy series are battle centered, it makes sense that the prologue would contain some form of battle. The problem with these battle prologues is the fact that the hero(es) and his foe’s powers are unknown to us and their introduction all at once risks exposition being delivered in info-dumps throughout the fight itself. Unless the powers or abilities are intrinsically simple, these battle prologues can be disengaging for the readers.
Being a common beginning for fantasy stories, an example battle prologue that springs mind is from Robert Jordan’s successor, Brandon Sanderson, in the prologue to his book The Way of Kings.
Whatever type of prologue is used the results are generally the same. The hero, or at least a heroic figure, will fall to the foe or vise-versa, shifting the responsibility onto another. How the defeat occurs is usually very indicative of the overall meaning behind the story. For instance, Lew Therin Telamon commits suicide after the madness brought on by his power drives him to kill his loved ones, showing that power in this world will come at a cost to one’s mental state, a common theme in the The Wheel of Time. The defeat may simply be time, whereby the historical record of the hero will be important. Another common theme is sacrifice, whereby the old hero gives his own life in an attempt cut off or pass on something that might help the new hero who will have to take up their burden.
The most common example of this fall is the death of a King or some other ruling figure, whether this be in Steven Erikson’s Malazan Book of the Fallen, The Emperor’s Blades, A Game of thrones or the beginning of Tad William’s The Dragonbone Chair.
The point behind these prologues is to show the source or an example of the troubles and conflict the protagonists are going to inherit. Making this source or example set at a distance from them not only helps to show the scale of the conflict, which is important for epic fantasy, but also to make a juxtaposition between the what is and what could be the reality for their settings once the foe returns. In a sense, when the old hero passes the torch to the new, it is commonly the case that they inherit even more darkness from the old heroes’ failures than light. However, it is learning to control this inherited light that makes for the majority of the heroes journey.
A common theme is that characters themselves are this light, as even the inherited benefit is that of the foe’s corrupting power, mostly recognized in J.R.R. Tolkien’s The Lord of the Rings. Although, one could argue that the beginning of The Lord of the Rings was more about hobbits and I’m thinking of the film’s prologue, in which case, that’s a really good example of a prologue in a fantasy film.