DM: Outside the wedding hall, you see the barrier start to crack like glass. All of the changelings outside are just ramming into it, over and over.
Twilight Sparkle: Welp, guess that takes it out of our hands- er, hooves. Off we go!
Rainbow Dash: Hey, we don't really need the bridesmaid dresses anymore, right?
Applejack: Now that you mention it, naw!
Rainbow Dash: Sweet! Off they go!
Rarity: Aww, but I worked hard on those!
DM: As you flee the wedding hall, you see Princess Celestia taking flight to attack. She fires down a beam of pure light, hotter than the desert sun.
Rarity: Nope! No dress is worth sticking around for that!
See? That's what I'm talking about, now just have her captured under a pile of charred changelings and boom, power mystique maintained.
...er, perhaps charred is a bit much for TVY7. Pilled up with stars around their head?
I wonder if the DM was quietly rolling dice to see when the changelings broke through the shield all through the wedding scene? Not a real player of the game, but the way DMs try to disguise something odd happening without the player noticing intrigues me a bit.
The easiest ways would be a static timer, with the description getting more dire as it gets closer, or it being timed exactly to be when the players reach/near their goal. A bit cliché on the latter option, but I've noticed most people (at least the ones I've managed to talk to related to gaming) are usually of the mindset to avoid adding more numbers and counters to track.
Personally? I'd either go with your option, or the first one I mentioned. I just don't have the experience to manage games at the moment, so I'm going to be waiting quite a while to run something cinematic like that
The way to go for that is going to depend on the players. Do they want a story that has the cliché things like timers that end just as the party gets there or do they sometimes want the subversion of showing up with 15min to spare. Or maybe them never getting a chance to avert disaster at all. It's also good as a DM if you know that they DO want the cliché but don't want to admit it. My players are often like that. Where they have the most fun is when they get to arrive just in the nick of time and I lie to them about whether or not that was guaranteed. A few meaningless dice rolls and some ominous descriptions after I make "Oh no I rolled a 1" reactions and they are into the fiction that there was ever a chance they wouldn't arrive just when they needed to.
You can't pull that too often or it becomes obvious of course but it's good to pull it out when the scene really calls for it.
Vampire? That's quite a shift in style and pace. Wish you great success, and try to keep mechanics from interferencing with the story, and use Narrator discretion to decide to obviate them altogether if needed... World of Darkness has some great things, but their mechanics often suffer from misalignment with the narrative intent of the games.
I've been reading about mid-18th-century warfare, and this phenomenon is generally written as "Commoners died when their betters fell out."
One of my favorite fantasy authors, Fiona Patton, specializes in these types of stories, and I've swiped bits of her worldbuilding for my games. The Gods are not just real, they take over people's bodies and minds. So while it may be an honor to be worshipped as the avatar of a god, it's also painful, consuming and sanity-rending - and even worse if the avatar is, say, a five-year-old child. There are plenty of cases where the Gods are fighting over a battlefield and their followers can do nothing but run or get knocked flat. And they know it.
Personally, when one of the demigods is a personal friend, it takes a lot of convincing to get me to leave them. Maybe the players aren't in the big leagues yet, but if you tip the balance in favor of your demigod, they can clean up the smaller problems after they win. Now, it's clear here that Celestia has no chance, and the elements are their only hope, but it's still upsetting to leave a friend in a jam like that.
The catch being that the demigod is their organizational superior and has ordered them to leave it to her. They have both a secular and religious duty to obey. If Celestia were actually a goddess, that is.
(Anyway, tomorrow I'm starting the first Vampire: the Masquerade campaign I've ever run/played and I'm quite nervouscited.)