Quartet world
Setting is modern day.
An urban legend in this time goes: some people in the world are able to shapeshift into animal forms (including mythical animals) that also may have powers (more flexible than "elemental" but not what I'd call "magic").
They do this with the aid of metal pendant things. Through fate/destiny/whatever it's called, the pendants find their way to their owners.(throwing it away results in a series of coincidences that make it land back in the person's possession) Only the rightful owner of the piece of jewellery can shapeshift with it. The form of the animal's determined by the owner's first shapeshift, however they imagine themselves to look like the first time. Although the pendant places some restrictions by looking vaguely like the animal they're supposed to turn into.
Animals can range from the absolutely mundane (somewhere out there, someone's a cockroach.) to mythical beasts sooo it's sort of a matter of luck. Some say (i.e. I never fixed it) it depends on the person's heart, that sort of thing. There can be duplicates (like two different people are fire dragons) but no one looks exactly like anyone else because their imaginations are different.
It's not particularly useful in modern life since well..potential destruction of cities much? The pendants usually end up being jewellery. Not necessarily as pendants either, I suppose some people wear them as earrings or something.
In one variation of this universe, these shapeshifters are sort of kidnapped to some isolated place they can't escape from, that's run by some weird dude who makes them all fight each other to establish some kind of hierarchy/organisation, saying it's to fight some large enemy who poses a threat. The weakest ones are killed off because they consume resources to live and aren't worth keeping alive. (they can't be sent back either because this is all top secret. or something.)
In some variations of this variation they're eventually paired up to work together for various reasons.(mostly just an excuse to shove the characters together)
It's not very well worked out, so it's not concrete :'D But this is the variation I throw them in, mostly.
At the risk of sounding stupid, expanding that variation I put these 4 kids in. This is incredibly old (beginning in...2003 thereabouts? Maybe later or earlier...)- I haven't thought up of any new place to put them.
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kidnapped
The kids, along with other shapeshifters (who apparently are mostly little kids to young adults) are kidnapped by some unknown organisation to some unknown location for some unknown purpose. (...I guess I never worked those out.)
The details change frequently, but some things remain constant:
1. It's a dangerous place to be.
There are regular battles organised between the shapeshifters, and losing at the lower levels of the competitions almost inevitably means death by some painful means. Or being whisked away to the bowels of the huge building they're kept in where their life energy is drained for power. Mostly electric power. (where did you think they get their electricity from?)
The four top fighters are termed the Kings. They get treated with awe and respect. Generally, you wouldn't want to come up against them in a fight. And guess which four people hold these positions...
Defiance is also punishable by slow painful/painful and bloody death. I'll hold back on the details, but suffice to say I was a rather morbid kid...the morbidity never really went away, either.
Anyway, it's listen to the boss or die.
2. The boss.
There is a mysterious boss who runs these things. He shows up at announcements and whatnot and generally reeks of suspicion. Most of the time he's a cheery sunglasses-wearing guy who can happily order people dead, even if they're tiny kids. And watch the process without batting an eyelid. But you wouldn't know those kids are dying- that's a secret, so all you see is a cheery guy who gives orders and doesn't take questions.
It's not known if he's a shapeshifter himself (in fact I didn't think of it until now), but probably isn't. Somehow he has a lot of power, though. Something prevents everyone else from ganging up on him and killing him. It might be that no one knows where they are (whether they're even on Earth is a problem here) and he's the only way home, or that he holds in his hand the whereabouts of that loved one who went missing last week (probably dead).
All the missing people do add up sometime, so some excuse has to be given to the masses (apparently there are a lot of shapeshifters...) - usually it's said that they went somewhere else. Relocated somewhere, or sent home. In the latter case, some people think they can go home by losing, but the smarter ones think it's fishy and don't take the risk.
3. Partners.
In most variations, each shapeshifter needs to find him/herself a partner. Partners supposedly boost each other's strengths, resonation of powers, that sort of thing. Read this as excuse for people to get romantically involved (and more) with each other.
Sometimes the partners are forced, sometimes it involves running through a dangerous maze sort of thing (more dead people) where at the end you meet your partner. (someone in a control room moves parts of the maze around such that people they want to put together meet up in the end - no idea why they'd go through this trouble.) Or another variation of the maze is that you have to find your partner before you leave it. Whichever way things happen, the kings always end up with each other. (curiously, a girl/girl pair and a boy/boy pair.)
Also, boss says: Those with no partners can come and have a little chat with me.
You know what this means...
4. Enemies.
They're all supposedly being trained to fight some major enemy. I never worked out what sort of enemy it would be to justify keeping so many people in such a place. Maybe it's a bunch of other shapeshifters coming from another place, trained in similar ways...rather like Bokurano. Or maybe some aggressive aliens planning to attack. Or there really isn't any enemy and the boss is using them for selfish purposes, harvesting their power or something...