There isn't actually a lot of preparation leading up to Atlantis - at least not on the part of any of the workers. The Ringmaster, it would seem, has been preparing for it on her own, and the first anyone will be hearing about it is when they wake up with the carnival inexplicably underwater on the morning of Day 44. Also, they will have magically mutated into having fish parts, mermaid style.
Just what kind of fish parts they end up with is infinitely variable, and doesn't even have to stay restricted to fish, technically speaking. Any aquatic creature will suffice, whether that be dolphins, seals, crabs, shrimp, eels, squid, or anything else that would fit beneath the ocean. Even if you choose to base your mutations on a mammal that regularly requires air to survive, they won't need to go above water in this case. Everyone can breathe underwater equally, and so other physical changes will be mostly aesthetic and/or just affect the way they swim.
Character cannot ICly pick their mer forms - what they change into and how much they change is entirely an OOC decision. You can go for the full fish tail, or just webbed digits, gills, and a swimmer body. It doesn't really matter, as long as they function in the ocean.
Naturally, the suddenness of all this may be quite alarming, and it will seem that the decision to come here was very impulsive on the Ringmaster's part. Even if your character doesn't usually sleep, they will find themselves having drifted off during the transition period between there being water and no water. The Ringmaster will explain to those concerned that everything is safely waterproofed via enchantment, and all of the carnival equipment will work the same as it did before. It's just... underwater now. Huh.
Atlantis is an underwater city within that the Ringmaster has describe as a "primordial ocean realm" that has no surface and no land above the sea. It is apparently connected, in small amounts, to many other oceans across the multiverse, and ships and other ocean vessels have a tendency to sink into it, Bermuda Triangle style, allowing the merfolk there to gather sunken treasure from across all of existence. The city itself is shockingly grand and elaborate for something that was apparently constructed underwater.
The Ringmaster has indicated that merfolk are some distant cousin to the fae, and the magic within the place will feel familiar to what exists within the carnival. Along with the merfolk inhabiting it, there is a large variety of other sea creatures and monsters that live in the surrounding areas. It's not clear how far the ocean stretches, but it feels effectively infinite. The carnival has located smack-dab in the middle of a beautiful underwater park within the city, and has physically merged with the local realm as opposed to remaining separate as it usually does. Tall underwater plants have replaced the barrier of trees.
► KING TRITON: The powerful king and all around boss of the merfolk and the city of Atlantis, he is apparently the lord of the realm and the most powerful creature within it. That's what the merfolk say, anyway. Apparently he is an old connection of the Ringmaster's, which is why they have permission to come and perform for his people. In this context, Atlantis is fully aware that people from another world are coming to hang out and party, so none of them will be surprised by the visitors, and are in fact looking forward to it. King Triton himself is stern in a fatherly sort of way - which makes sense, given the fact that he has what is probably hundreds of daughters of varying ages. He takes the security of his people very seriously against outside threats, and tolerates the Ringmaster with a mild amount of reluctance.
► MERFOLK: As variable as the carnival worker's potential changes, merfolk encompass most ocean life, aesthetically speaking. While most of them average out with things resembling fish tails, you will also find a great degree of biological diversity that is slightly more rare. In general, merfolk are curious and straight-forward, in contrast to their fae cousins. They enjoy seeing new things, and are as excited to explore new people as they are to explore a freshly sunken wreck. They consider outsiders to be a major curiousity, and in crowded social situations, they may end up competing for a carnival worker's attention - especially those that look most dissimilar to them.
► THE SEA WITCH: If there is one thing most merfolk will specifically warn visitors of, it's to avoid the Sea Witch. Known to be a sorceress with particularly devilish abilities, the Witch has been actively antagonistic towards King Triton for thousands of years, with no signs of stopping. The common merfolk will attribute all kinds of wrong doings to her, from stealing souls to ruining this year's kelp harvest. What part of the stories are true will be hard to make out from the stories alone - including the parts that suggest she has the power to grand one's deepest and darkest desires.
► TAMATOA: A beast that is absolutely secondary to the Sea Witch in nastiness, but still gets complained about all the same. Tamatoa is a giant monster and all around shit-crab that makes a point of snatching up anything golden that he can find to adorn his lustrous shell. More of an annoyance than a major threat, he has often scavenged away treasure before the merfolk can get to it, resulting in no shortage of hurt feelings. Also, he will casually eat just about any life-form that can fit into his mouth if they cross him, and sometimes even if they don't. Extremely vain and predictable, he has a horde of magical objects that he's collected over the years but has never used - mostly just because he's a fucking giant crab and what do you expect him to do with them. Note that he is not the exact same rendition as the Tamatoa from Moana, meaning that he still has all of his legs.
Merfolk love trading, and as such there will be opportunities set up for workers to either spend an allowance given to them by the Ringmaster, or to trade their own belongings in the various markets inside of Atlantis. Also, with the right amount of smarts and caution, it is also theoretically possible to get stuff out of the Sea Witch or Tamatoa if you play your cards right - that is, if you're willing to risk having them eat your body and/or soul.
If one gets friendly with the merfolk, there is also good odds that some of the workers will be invited along for some treasure hunting, that could result in some random loot and various shinies as well.
Characters may optionally keep (decided on an OOC basis) one uncommon or common change related to their mer-form. Essentially, this will just be them leaving Atlantis at the end, and having a certain part of their underwater look stick with them. Changes are also theoretically available as part of random dealings with NPCs, but those will be dealt with as they arise.
Beyond that, this is a PLOTTING POST, which means you can use it to make plans! The game chat is also a great place to discuss plots, so we welcome you to pop on in if you haven't already! A mingle log will be posted this weekend.