A young pairson in Twinwirld grows up with a natural sense of being just one unit, even though twey consist of two disconnected parts. "Every dividual is indivisible", runs an ancient Twinwirld saying. All sorts of conventions in Twinwirld systematically reinforce and lock in this feeling of unity and indivisibility. - Douglas R. Hofstadter, I Am a Strange Loop

60% of the world is identical twins

This is sort of based on a thought experiment that comes from the book quoted above...as an introduction to the idea, here is how the thought experiment is introduced:

Once, some years ago, I concocted a curious philosophical fantasy-world, to which now, with your permission, I'll escort you for the next few sections. Although back then I didn't give the place a name, I think I'll call it "Twinwirld" here. The special feature of Twinwirld is that 99 percent of all births result in identical twins, and only 1 percent give rise to singletons, which are not called that, but "halflings". In Twinwirld, twins (who, as in our world, are not exactly identical but have the same genome) grow up together and go everywhere together, wearing identical clothes, attending the same schools, taking the same courses, cooperating on homework assignments, making the same friends, learning to play the same musical instrument, eventually taking a single job together as a team, and so forth. A pair of identical twins in Twinwirld is called, rather inevitably, a "pairson" or a "dividual" (or even just a "dual").

I'm not using the same terms or even the 99% figure (it's anywhere between 0-99% depending on where you are in my version; ~60% on average), but this universe is pretty much this thought experiment as a world, or is intended to be anyway.

world

Established facts/some glossary
- it's known that twins naturally come with magic capabilities
- they're linked in mystical ways. something like that
- non-twins are called singletons
- a pair of twins are referred to as a single unit - they have a single official name though they commonly have nicknames that other people use to refer to each half
- at the moment I refer to the ritual that turns one into a creature partner as conversion, twins that have undergone this ritual are a converted pair
- converted creature forms are passed down in the family - children's forms will resemble either one of the parents' (if both parents are twins)
- it's not real genetics, if a form is not passed down it will not show up later down the line
- after the conversion, the newly converted pair is generally apprenticed to an older pair that teaches them how to use magic
- after they've learnt some of it they can choose to continue studying, get to work, or travel...though they're generally expected to work. They're almost never allowed any non-magical jobs since it's considered a waste of talent/unfair for singletons

History of the land and its twin customs
- there's some apocalyptic creature living deep underground
- every [insert long period of time] it surfaces and does apocalyptic things then goes back to sleep
- one of those apocalyptic things is a plague that leaks out of the ground and infects animals/monsters and people (see below for details)
- it's prophesied that a fenmine (a serpent-like creature with heads on both ends + wings) will rise to defeat the creature someday
- in the last cycle, a pair of twins worked out a magic ritual that would turn one of them into a creature, figuring that this could help them fight the plague by defeating infected monsters more effectively, and possibly even fight the apocalyptic creature
- this pair was Fenmine
- once it was proved that they were immune to the disease people set to work converting all their twins, the twins organisation/association was formed at this time to assemble an army of twins
- long story short they fought a war against the apocalyptic creature and successfully sent it back to sleep much sooner than it would normally have
- Fenmine was hailed as a hero, the fenmine became the official symbol of the twins association
- the story became legend, it's told as a fairy tale (with much embellishment) and some places treat it as fiction
- generally people assume that the creature, according to the prophecy (which didn't say anything about permanence), is gone for good
- there was still a need to mop up the rest of the infected monsters so the conversion ritual became some sort of a tradition/obligation for anyone born as a pair of twins
- in modern times there are much fewer of these (so much that it's a bit more like an urban legend, you only see them in places that people don't normally go) but people do the conversion anyway, because there are many uses for magic
- because the old need isn't really there any more some people in modern times (especially those who don't believe the legend) refuse to undergo the ritual

Plague details
- creatures can host the disease without dying (it turns them rabid instead and makes them attack people), they're the main vector for the disease
- it's fatal for non-twin (singletons) humans
- twins have some resistance, but the survival rate isn't 100%
- converted twins are completely immune for reasons that are currently unnecessary detail
- if one half of a pair of converted twins dies, the remaining half will be infected like a normal person (if the remaining half is human) or become a rabid creature (if creature)
- the plague/apocalyptic creature generally leads to mass deaths and is probably why there can be such a huge difference in technology levels between cities/countries - the human population just doesn't get big enough and a lot of time is spent in between cycles just reconstructing everything
- it also explains why twins are so common
- initially the plague is almost immediately fatal (takes effect in minutes to hours), but as time goes on it mutates? and leaves human hosts alive long enough to spread to other humans

Apocalyptic creature
- maybe it's the use of magic that keeps the creature alive
- in the sense that as long as people continue to use magic it'll continue to exist
- so it'll never go away
- since magic's its sustenance, whenever the magic-using population grows too big it'll wake up and go on a rampage
- it can be temporarily calmed by defeating it i.e. hit it until it passes out
- it feels like the planet's doomed to be destroyed eventually lol...eventually the population will grow too big and it'll grow too strong and it'll send everything back to square one
- that will probably be several billion years in the future though
- I'm reminded of the line from the Kino no Tabi story that I like the most though: "As long as we have a will to live, we won't die so easily."
- so I guess that'll just be part of a longer cycle where civilisation rebuilds itself...at least, it's something that's irrelevant to the current story

Magic
this is the most headachey part of working out any world based on magic, for me at least urgh
- the human twin can charge their creature twin with an element, which alters their appearance/abilities
- long term use of particular elements will make some of those changes in appearance permanent
- "element" is determined extremely loosely, the idea of what's considered an "element" is a topic of research in-universe
- fighting's generally carried out by the creature twin, in Pokemon-ish fashion
- the human twin can use offensive magic, but it's much more efficient to buff their twin to fight
- magic consumes energy, sort of like the auto-replenishing stamina bar when running in a game, though the replenishment takes a while (usually a good night's sleep is enough to recover spent energy)
- magical work is mainly about research, development/powering of technology that uses it, or field work
- research generally concerns the range of elements, how to use it efficiently, etc.
- technology is about amplifying/efficiently using magic for mass consumption (physics doesn't apply), usually this only concerns large cities where twins can get paid to use their energy to power their inner workings
- field work is exploration and in more remote areas, cleaning out the remainder of infected monsters from the last apocalyptic cycle (work that no one ever hears about)