Something about a conversion bureau scenario that plays out similar to the (Biblical) Apocalypse from the "Darksiders" video games. As in this is how Earth gets it's first taste of Equestria and it's magic:

and it gets this extreme. people flocking to churches in a vain attempt to seek shelter as Celestia and the Villain's forces battle it out in the ruins of their once grand cities:

It would also be based on what I call "fantasy" apocalypse in which various magic or creatures from fantasy worlds invade and destroy modern civilization. Reign of fire being one example:

another being, Hellgate: London:

Basically, this scenario is a product of a war. Either two things: a war from Equestria spilling over into the Human world or Equestria's crusade to stop a villain's invasion to conquer Earth and their enslaving or genocide of an innocent species. Both cases are all the work of a Villain while Equestria remains the good guys.

one such example of a crossover into the human world would be the Crystal war from The Cutie Re-Mark:

spilling into the Human world after Sombra shifts gears and sets his sights in conquering Earth. The Battles between him and Celestia's forces inflicting this level of apocalyptic destruction upon human cities.

If there is any reason for the Villain to invade the Human world, it'd be because it's easier to conquer than Equestria and would be eager to make it their new domain and make it's natives suffer. Because of course it'd be in line with their ego and personality. They're evil after all.

Although the Villain's side in the war is defeated. The aftermath of these conflicts result in an outcome where Earth ravaged by their warring. Human civilization and infrastructure such as a cities totally ruined and reduced to unrecognizable wrecks as they became the site of major battles in the war.
\

In areas not affected by the War, they came under siege by wild and untamed magic from places such as the Everfree forest. While the magic itself wasn't lethal to humans, the various invasive flora and fauna from those untamed places were. rapidly spreading, devastating infrastructure and wreaking havoc on native ecosystems:

What ever remnants of Humanity that survived the conflict and were rescued by Equestria were left destitute and miserable. Having lost everything and everyone they held dear in the war and left with no choice but to accept Equestria's relief efforts as well as the offer to resettle in the relatively stable Equestria as Earth was rendered unfit for human life from said weaponized and untamed magic. Equestria clearly in guilt over the immense collateral damage inflicted in their crusade against the Villain's plot and their attempts to save the Humans.

And that's when the Conversion Bureaus are established.

well I would be down to reading that.

7846569
I mean it'd make sense for the unreforemed villains in the series to shift their goal towards conquering another dimension and kicking puppies (doing something extremely and needlessly evil, cruel or very mean for no apparent gain,) in the process. Humanity being the puppy. Humans have no magic to defend themselves, making them easier pickings than Ponies. So most of the apocalyptic damage and most of the Human Deaths seen would be from them as Equestria would enter the human world much later.

7846772
I could also see the majority of the human survivors placing the blame onto the ponies as well since there came from the same realm and probably cause some of damage during the fighting, you could even have wild magic leaking from the everfree thu the dimension gates leading to even more chaos. It would be a solid reason for humans to hate ponies since there need someone to blame for the cataclysm and humans can be quite dumb and quick to judge when we angry and grieving.

I think Grogar invading earth rather than sombra is better. We pretty much know what sombra is capable off, but Grogar? Zero clue. (Assuming he's not the same grogar for G1).

Your story idea however, is fantastic. Though, it would be better if TCB isn't the main factor of the story. I think humans adapting to their new environment would be better.

7846818
how about the others? like storm king, chrysalis and tirek?

7846802
so what would the survivors do to the surviving equine population of Earth then?

7846837
Well many things I would think. Our response to the end of the world can manifest in many different ways, some will see God and start cults, others will use it as a chance to gain power or some will try to just get on with life. There will be many different groups each with there own opinions about the equines and there own methods of dealing with them. Some will choose violence, others will try cooperation and friendship while there are some who would have nonething to do with them. It why I like this idea so much, it has a lot more freedom for worldbuilding than the standard tcb has. Like a off brand version of fallout with some of the zone chaos thrown in.

7846840
I meant IRL horses native to Earth. Would Human survivors vent their anger and loss on any Earth Horses that survived the war?

7846841
.....huh ok wasn't expecting that, well I could see a movement or two base around it, either as a form of religious sacrifice or just some form of anger release by roaming gangs or zealots but I guess most of them would probably be kept for labour or in dire cases, food, though I could see ponies be somewhat horrified by the supposed slavery and consumption of, I guess in there eyes, their ancestors (sort of like apes with us I guess). hell, you could even have them mutate, have them over generations become more pony like as equestira magic start to change them, make them capable of speech and even giving birth to earth born pegasus and unicorns.

7846852
as for humans blaming ponies, i could see that happen with Chrysalis and King Sombra since one can shapeshift into ponies and the other uses mind-slaved ponies, but for others such as Tirek or the Storm King or even Grogar, there would be a distinction between them and ponies.

7846858
aye that is true and I suspect many would. but you have to remember two important factors : propaganda and human stupidly, throw in a global blackout of communication due to the war and you can see all manners of beliefs systems and hearsay becoming facts in people eyes. all it needs is the right person to gain a crowd attention and before you know it. old sun butt is the devil and burning a horse every Wednesday will free you from sin.

7846875
the only ones making the distinction would be those in the presence of the ponies. knowing that the ponies clearly are the good guys in their situation.

what about moon butt?

7846820
The storm king is already dead in the movie. Chrysalis? She could just take the humans and replace them with changelings. But knowing her? Her arrogance might screw that plan of hers. But even she tries a frontal assault, her changelings are dependent on numbers and can be taken out with the human arsenals. The changelings could damage the humans sure, but not enough to earn victory.

7847051
For the storm king part, it could happen in an alternate timeline or universe made for this CB scenario. the MF'er conquers places for the sake of conquering so it would be in line with his Modus Operandi to conquer the Human world. The title of "conquerer of worlds" would be so alluring to an egomaniac such as him and this would embolden him into researching ways to open other portals to conquer the multiverse.

7846991
lets just hope the ones who can't don't spot the ponies at a distance with scoped rifles :). though I could see alot of tension and mistrust at first for the non hostiles humans, it would take a long time for humans to warm to them and this is before we get to the bureau. i can see alot of people not liking the idea of them, could be seen as a form of extermination to finish off the survivors or them trying to hide there mistake from the rest of equis since the other species/ nations would not look to kindly upon pony kind and there actions against the new worlders, depending on how much damage there cause in the war (a couple of newspapers printing the wrong stories and now the whole world thinks your a mass murder, it happen to the Vietnam vets, it can happen to sunbutt)

as for moon butt, i guessing she be forgotten about just how the rest of pony kind did ;) nah i kid. she probably be working overtime in the dream realm, trying to bridge the gap between the two species. tho this will likely make the zealots hate them more, seeing it as a form of attack or attempted mind control. for others it might just scare the shit out of them as a giant talking horse fly's at them with the canterlot voice, who can say for certain.

7847067
Forgot to mention the storm king. But he is MUCH weaker than sombra though.

7847218
imagine this war from the perspective of a horse rancher.

7847463
a war against him would be more like a conventional war wirh human military tech fighting against his military tech.

7847478
terrifying with a hint of madness, after all who do you think would be the ones that would start the horse cults in the first place :)
it be like those crazy pastors you see in the back states in the us. there get a visit from luna in there dreams and boom, there preaching to the commune "I have seen god face my children and it is blue!".