And the poll winner is
Chrono Trigger, with the disaster being Lavos: an asteroid impact and the living being within the asteroid emerging billions of millennia later, creating a supervolcanic eruption and a verneshot (displacement of material into the upper atmosphere, for it to rain back down equivalent to an asteroid impact/meteor storm)
I'm going to be brushing up on research on this and writing it up as best as I can over this week - like some of our disasters (e.g. the close runner up of Xenogears' Disc 2 with nanomolecular gray goo/mutation zombie apocalypse) this is one that's mostly speculative/hypothetical - even the closest analog we have in real life (the Chixculub impact) is something no one living thankfully experienced, although there's smaller impacts that have been (Tunguska, Chelyabinsk) and a lot can be inferred from those and calculations from nuclear weapon yields and the like.
Similarly, no supervolcanic eruption has happened in the real world since Krakatoa, so while it's somewhat more easy to model from living memory than an extinction level asteroid (and we do know more about volcanoes and how they work than asteroids and comets) there has never been a true verneshot within human existence (which would require a lot more power than even Krakatoa provided, it would likely need at least a level of the Toba catastrophe)
Congratulations to the people who voted for Chrono Trigger, and I shall have this up sometime within the week: I'm going to do some more research, update what I know from what real events and science we do have, so while I'm obviously speculating, I'm not JUST regurgitating something like Supervolcano or History Channel Doomsday and covering the same territory they did but for the setting of Chrono Trigger. (And unlike Doomsday, I promise there will be NO CGI butt plugs presented as Balanced Rock at Arches National Park. Not even joking, I just saw that episode again...)
Also, to be truly pedantic, since Lavos is a "living being," this would also qualify as an "alien attack," but I'm going with "asteroid, supervolcano, and verneshot" since that's the apparent effects and how it did its damage - it didn't say release armies or biological substances, didn't intentionally terraform aside from the results of its arrival, didn't take over existing power structures (but did get found/used by them) and seemed to do most of its damage just from being an impacting object and then from coming out from the ground.
Although that might be an interesting point for the write-up about true accidents/purely natural disasters/competing access need events: e.g. disasters that no one could have prevented because everyone/everything involved was operating exactly as intended/just as it does, and the bad consequences could not have been reasonably foreseen/were foreseen way too late, or it wasn't really anyone's specific fault but just one of bad placement for population/competing needs - e.g. in this fictional scenario, a planet devourer that just needs a solid meal, unfortunately the place is populated.
