You guys want a major pro tip that's going to make me sound elitist but will save all of your readers' lives and possibly your own?
If you have a prophecy, or a huge long story about an OC you came up with that's supposed to introduce us to them without having them on screen, and you want to make that your prologue, consider this: don't. I have spent the last 3 days trying to get through a 500 word prologue that's a backstory for an OC and it's so boring; I'm tempted to skip it, though I'll probably miss important information. "But my intro is really good and important!" someone, somewhere, will probably say upon seeing this post, and I'm sure they think so, but...most of the editing I've done for other people is high fantasy, which uses this trope a lot, and, of the few dozen times I've seen it, it's never read well. Ever. I'm not saying "don't ever have a prophecy or a story others have told about your OC"--y'know, write what's good for your story. But...well...here, let me just explain the reasons it reads poorly and how I think it would sound better:
Reasons you shouldn't do the thing and possible solutions:
1) you're giving away 90% of your plot (mainly the problem of prophecies) - I'm not going to be surprised when you break out the whole "oooh, magical prophecy" routine because you've already told me everything. I already know the entire story with your intro. What is my motivation to keep reading at this point?
Possible solution: save your prophecy until a later time to reveal it, that way it'll be an actual surprise. You could break your MC with this information or rebuild them with it. Use it as you would any other important plot device.
2) you're info-dumping us (mainly the problem with revealing a character's backstory before the story starts) - you know how some people hate reading descriptions? This is exactly why. You're throwing a bucket of info at me, expecting me to be able to piece together what's important and what's not, and expecting me to somehow come to like this character in the process? I don't, because I still have no emotions attached to this character and I don't even have the urge to keep reading this story because of it. It's just too much. And it's usually a sign that you have no plans on building on your character.
Possible solution: just...don't...do it? That's not a good solution. Okay, real answer: reveal their background info through the plot. This seems straightforward and, yeah, it's exactly how it sounds. Have little things in the plot or their environment, or conversations with other characters, or even just memories, reveal this info instead of you throwing it at me. That's literally all it takes. Let us get attached to the character before you bring on all the bombshells. Pepper the backstories through the series and readers will adore learning about this character. Otherwise we have no reason to be invested.
3) it's usually a sign of an under-developed story - that's...really it. Like, really. It's a sign you haven't developed your story enough, because you're trying to inflate things with artificial drama. The thing is: we don't care. Does that sound callous? Yes. But, if this is the opening of your story, we know nothing about the characters or the world and we have no attachment to anything you've written. We need a living, breathing world to connect to, not some random Wikipedia article.
Possible solution: okay, so...if you absolutely have to do this and start your story this way, then please, for the love of Avo, write it well. Most of these read somewhere between extremely pretentious and that bland, textureless way a cheap tv dinner tastes. Make sure your world feels alive and vibrant so that, while we're reading about your prophecy or your OC's entire life story, it feels like I'm in the world and not just staring at some boring text book that cost me $600 and will end up not even being required reading. Give it life, not a painful amount of dust.
...okay, that ran on longer than I meant it to. Fingers crossed I'll make it through this ridiculous intro.
