One extra allowance for fantasy writers is the convention of the fantasy prologue. Namely, a block of text, usually half a page to a couple of pages long, which stands on its own before the novel’s official opening.
A prologue used to serve as a sort of introduction, providing background information. But most prologues nowadays are short dramatic scenes, disconnected from the official opening and having no immediate relation to the main story.
One use for this kind of prologue is to provide a burst of excitement if Chapter 1 of the main story doesn’t launch straight into dramatic action.
Side-note: when you browse a fantasy novel in a bookshop, do you glance at the prologue or at the first page of the Chapter 1? I tend to go for the first page, then the prologue afterwards if the first page catches my interest …
Probably the most important use for this kind of prologue is to spread out the size of an otherworld. If a fantasy begins in a small corner of its world (for reasons suggested in Section (viii)), a prologue can let the reader know there’s a wider story and bigger events to come. It’s like a teaser or trailer, jumping out to a far-distant metropolis or an earlier period of history. We know that story will eventually expand to include a relevance for this disconnected scene.
‘Eventually’ may be a long way away, though. The connection to the main story may not emerge for a hundred or several hundred pages. Are readers seriously expected to wait all that time for an answer?
My guess is that such prologues soon drop out of the reader’s mind. When a prologue is disconnected and without context, it’s like a dream, and soon sinks away for lack of reinforcement. You know how a dream in the morning is so clear when you wake up? – then five minutes later, you’re frustrated to discover only a few traces remain; fifteen minutes later, it’s gone for good.
If a prologue is used for excitement or to spread out the size of an otherworld, this is no problem at all. The prologue has already done its job. In fact, it could be an advantage, because you don’t need to worry about pre-empting later story developments or giving away too much. As soon as readers get caught up in living through the events of the main story, you can rely them largely forgetting the prologue.
I suspect it’s more a matter of having to make the connection very explicit if you do want readers to remember back, a hundred or several hundred pages further on.
I haven't used prologues much in my own books - I'm not sure why not. Maybe because I haven't written any huge epic fantasies (yet!) I didn't even think of a prologue for Song of the Slums until my German publisher suggested one. The drama in the opening chapters of the novel is all about romance and an engagement-that-isn't; my German publisher felt that this is lopsided as an introduction to the novel, which contains as much action and adventure as my two previous steampunk books. Not just 'a book for girls'. The suggestion was to give an earlier 'signal' with a prologue, an action-packed extract plucked from later in the book that would show Astor and Verrol acting in tandem (as happens after the opening chapters).
It was a great suggestion, and I did want to convey the right balance from the start. There was another benefit too, if we could find an extract that would foreshadow the rock band theme that becomes central to the novel, but takes time to get under way. The perfect extract turned out to be the scene where Astor plays the drums for the very first time. Both Astor and Verrol are involved; it's a high tension episode because their survival depends upon it; and it's a key moment, perhaps the key moment, in launching the musical theme.
I have to start thinking more about prologues for myself, without waiting to be told!