I'm having some difficulty deciding what time of ending/final battle I want to write. The issue is that a main plot piece is the characters trying to stop someone from opening a prison and releasing a being that could destroy the planet. Should the battle end before the villain is able to open the prison or have the prison be opened and the main heroes have to stop this being? I've been building up that the being in the prison is super powerful, but so is the main villain. Would it be more exciting for the characters to fight this being and stop the apocalypse or have the final battle end with the main villain's defeat? If that makes sense!
Unless you plan on showing this villain in some kind of flashback or video or something during the course of the plotline, it is always going to feel a bit ambiguous and anticlimactic to stop a thing that is only perceived by the characters as powerful. Without image relation, the reader probably isn't going to be that concerned, and honestly maybe disappointed, with a villain that doesn't really get to show much face value. Implied evil and destruction is cool and all, but it really isn't the same and is less effective if we don't really have much of a frame of reference. Just my opinion,but I'm sure I'm not alone. Spice it up with some failure on the protagonist's side.
You build your reader up all this time to imagining this amazing, powerful being inside this prison and then they never get to see it... Well, imagine when you go to a restaurant, look at the menu and you got your heart set on a particularly delicious dish, just to be told, "Sorry, we're out of stock for that one at the moment." Only in your case, you've strung your reader along for 80,000 words or so... What are the chances they would throw your book at the wall? I'm not sure the force is with you on this one.
I'm not sure there's an "on the other hand" here. If you've been strung along an entire book just to realise you have to wait for another entire book before you get to see what you've been built up to see... I honestly don't see the attraction in that. I'm sure there will be readers who would plough on because they've invested in so much already, but I doubt it would be done happily. Happily being the key word. If you have a disgruntled reader, even if he does finish your book, you can be sure he won't be returning for your new books. It's also just a very cheap way of getting a sequel. How do I, as the reader, know the next book would be different? If the goal is the same (it's still "stop the prison from opening) in book 2, so it's just the same story on repeat. It's one (of many) reasons I disliked Girl of Paper and Fire (but that book has way deeper issues than just the ending). And I'm a huge fan of Hunger Games but I disliked Book 2 - I found it unnecessary as it's just Katniss going through the games again with different challenges and different characters. Some of the events needed to start the third book could surely have been condensed into what is now the third book. Plot repetition so you can have a sequel is not a good strategy. If it's a sequel you want, better would be if the prison is opened and now they have an even worse thing to contend with than in the first book with the main villain. I'm really not seeing how not opening the prison would make a better sequel?
I really think it all depends, particularly on what your story is actually building to. Is it building to defeating the unleashed evil or actually preventing it? Because I’ve never been disappointed by stories building up the horrors of nuclear holocaust or inevitably devastating war which the heroes successfully circumvent at the last moment and never see what characters have “built up” as the repercussions for their failure. But I have been greatly disappointed in generic super hero skybeam battles, wherein they’ve failed to prevent the portal or release so now they have to defeat cliche big monster/inter-dimensional being or the world & populace are in imminent and catastrophic peril. I’m sure you can do it poorly or well either way, but to me is the emphasis on the prevention or building towards defeating the threat? How you handle it in the beginning will set the expectations for what will happen at the end. If you promise a battle with the biggest bad, then readers will probably be peeved if you deprive them. That’s my somehow still in circulation pennies.
It give it a twist like may be the being isnt all bad so the heroes have to open the prison to allow it to kill the super powerful villain... at the end with the villain dead the being gallops off into the hills giving the set up for book two when the heroes have to pursue it. Or the being is super bad so the heroes have to team up with the villain to kill it... or something