Events leading up to this location will be as normal. After Mainframe, the carnival will go into its usual travelling/transitional period for a few days, with no particular warnings or expectations. Normally this would be where I explain how the characters will entering the setting and what they will immediately see, but this is not a normal occasion.
Mostly because no one will have any idea how or why the carnival entered this universe - in fact, they may not remember entering anywhere at all.
Portland, USA is similar to the one of our modern earth in many ways, even if some of those are purely superficial. Importantly, it is the urban fantasy equivalent, meaning that standard city life is underlain with a complex civilization of 'supernaturals,' whose actions have the tendency to shake the world in unseen ways. It's a common formula in the multiverse, and some carnival workers may even come from a world much like it. The major difference here is this world in particular has managed to pull the carnival into its snare in an unprecedented way.
From the start of this visit an onward, the carnival will be rewritten into this world's rules - and in more than just a superficial sense. On day one, your characters will find themselves having awoken into completely new lives, with potentially no memory of anything that came before. How exactly this transition took place will start out unclear, and this will be a plot heavy narrative centered around investigation and the discovery of what has happened, and how those trapped here can leave.
Twenty five years ago, this world underwent a process sometimes know as "the Severing", in which a magical ritual of great power was used to severe the ties between earth and a number of otherworldly realms that had been cross-inhabiting it. The realms of faerie, heaven, and hell were disconnected entirely, stranding many of their members on earth with no way of returning home. The details of this ritual and its purpose are elusive, but its effects have spanned the globe.
Most supernatural stuff in this world is simply referred to as "magic", which is built upon an animistic reality in which all living creatures and all objects have some essence of "spirit" that can be interacted with. Casting and ritual magic is primarily centered upon esoteric means of interacting with these spirits in order to invoke various effects, while the magic of many supernatural creatures are considered to be innate traits of their species.
NOTE: This is a setting where magic is basically a "known secret", in that many people are factually aware of the supernatural, but it doesn't get talked about in public. Most people have probably seen something weird at some point in their life, but the supernatural is not officially recognized in society. Most of humanity does its best to try to pretend it doesn't exist, in order to avoid gaining the wrong kind of attention. So, you couldn't talk about doing magic on live TV, but there are a lot of places where you could get away with people seeing your supernatural traits if you keep to the 'underground.'
( A LOT MORE INFO TO FOLLOW )