I mean the question requires no explanation. I'd like to know your opinion on this. I honestly don't know (or remember) if the original TCB story that started this scenario mentioned a specific place, but there is certainly room in our world for more than one place. Heck, in some cases it could literally appear in the space of another country and create an international mess.

I believe middle of the Pacific and expanding outwards

Point nemo

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That's like, a super convenient location for them tho

Kinda like a fortress continent. I like it.

I'd place it in the exact geographic center of the Indian Ocean. Just close enough to populated areas and strategic choke points that when Xenolestia sends her forces, they can cause a lot of damage and possibly hold the world hostage by severely restricting its trade, but just far enough away from the superpowers that they can't immediately launch a counteroffensive.

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Pacific ocean, opposite side from Africa. And when Equus and Earth accidentally got joined at the hit and magic start flowing out like blood from a wound, Equestria ended up in a quantum state, existing on Equus and Earth at the same time. Also preventing any nasty tidal waves that would come from that water displacement on Equus dying from the lack of the sisters. It also mean it would be so time before the effects of magic on humans became apparently, and they'd send nukes to "take care of it" and Celly would use her sun magic (which itself is one big nuke) to negate the effects. And having to explain that it IS an cosmic accident and she's not engaging in territorial conquests or demanding transformed people give up their nationalities or religion.