So with the game set on the Australia layer, this means they usually have some degree of shade such that Drow etc. can live on the surface - which is good given you can't just keep digging down infinitely deep for your Underdark. You have the shadows of other islands, and you have light filtered through layers of water. Islands on a given layer are interconnected, allowing for ship travel, but going between layers requires long-distance flight: it's not unheard of, but it's rare enough that you don't have standard trade routes.
The players consist of: a Dryad Tree Speaker, a Xill that will later become a Moral Guide, a Lesser Cansin Conduit of Carceri, a Human Wizard/Eldritch Knight, a "basically whatever those dumb evil human things from BoVD are, but reflavoured" Fire Mage/Hell Rider and a Human Spirit Shaman/Stormcaster.
The first adventure was a pretty standard one designed just as the set-up to the cataclysm, which is in fact going to be so recent that it hasn't quite happened yet. They were hired by a Svirfneblin alchemist (of the "sells tanglefoot bags" variety) named Shnivnorshnipdip, to check out his cellar, because he recently discovered a problem: some tunnelling seemed to happen, and then he hired some people to check it out and they never returned, and a terrifying undead emerged – something which by his description sounds a lot like a wight. He managed to destroy it by using a lot of his crafted goods – alchemist oil, holy water, smoke bombs and tanglefoot bags. But now he has questions of “How many are there? What else is there? How did they get there? What are their plans? What happened to the other people, and can their remains be retrieved and put to rest? Was this done without council planning permission and am I in danger of being fined for these tunnels?”
So the party went down there and discovered a giant ant lion. A lot of the critters I placed were sort of placed with the idea of "they'll only attack if the players are looking for a fight. Deciding actually you do want to fight isn't enough to get a sudden surprise round, but for most of these things you can absolutely choose nonviolence." As such, they fed it a goodberry, using Speak With Animals (because it's not a vegetable or a mineral, so it must be an animal, fuck you D&D classifications). It didn't have much useful info, but gave them an envelope that used to be in the hand of someone it ate. It's just communication between two wealthy people, talking about a trade route and a possible jamboree or jubilee or tambourine or something.
Going down, they find themselves in a big cavern network, which was not mentioned, and there are even small bags and boxes of "stuff" stashed about. They encounter a wraith that has no interest in talking about the situation, but also has no interest in rolling high enough to hit with a touch attack, so it must be one of those useless WoD wraiths. 2-3 player characters lack any means to hit it as well, which is funny, but it is re-killed with fire. Later on, they discover wights, who turn out to be the previously-hired adventurers (minus one, who was carrying the letter). They still remember their prior existence and don't want to be eternal undead monsters, and ask to be killed so they can later on be resurrected. They also write down messages for their next of kin, so they can be notified in the meantime what with Resurrection not being the kind of thing 6th level characters can do.
The party also find an otyugh, and feed it, so it wanders off. Goodberry has so far been the MVP of this adventure. And that's when a sudden earthquake hits, and the place is in danger of caving in. They rush up to the cellar, find that new tunnel walls have been made, such as into a sewer, fires have started, and the stairs that lead out have caved in, but also water is leaking down so maybe the fires won't spread. It doesn't matter as they summon water elementals to put the fires out, then use wood shaping / etherealness to escape to the surface...
Where they discover the sky is falling. A huge amount of water from an air river has just fallen and smashed into the town, damaging buildings, but the earthquake was caused by a ship falling a few km and hitting the town. That's right: BOAT WENT BINTED.
I'm not a physicist so I don't know how bad it is when big things fall these distances. I mean I don't even know the mass of a ship, let alone what velocity it hits. So it was just "local earthquake".
And that is when they see in the distance, probably too far away to see*, an island falling from the sky and smashing into their continent in the distance. The session ends there, and next session they get to see how far the devastation reaches locally, check on things like the locals and "the nearby forest which has the dryad character's life tree" and figure out to what extent I started with a premise of "boat goes binted" and worked backwards to make a narrative for it.
*So sticking with the Australia analogy, the party are roughly in Coober Pedy, and an island roughly the size of Java or Cuba has fallen some 5-ish kilometres, hitting somewhere around Dubbo or Orange. The only Earth Impact calculator things I could find were all focused on asteroids/meteors, which tend to be a lot smaller but also move a lot faster, I'm pretty sure things like First Impact or Tunguska were moving at speeds where you talk about it in tenths of the speed of light. And results from those things suggested it could be anywhere between "750x Tsar Bomba, and nuclear explosions are actually pretty small on a planetary scale, you probably wouldn't even see the explosion from this distance" to "your whole continent is obliterated". If other people have better calculations for this sort of thing, I'll look, but as it is, it seems I have freedom to decide how bad it is.