They are parasites. Incorporeal, insidious, efficient. Individually weak, but drawn to power, magical more than mundane, and to those who seek it. Their influence is subtle at first, inclining their host to ambition and delusions of grandeur, amplifying desires and squashing qualms. They can enhance the magic of their host, but it is always temporary and always at a price; a slow leeching of vitality and power. All these traits are magnified if the host grows desperate enough to willingly accept the catalych's empowerment. And a Guest that has spent long enough studying its host always knows when the time is right to make such an offer. 

The plan is simple: Infect those with power. Steal it, wield it, grow and multiply. When the time is right, gather in secret and open the way to their masters in the Well. 

That was the plan, anyway. 

The Guests are drawn to magical girls like moths to flame. This one identifies a suitable candidate and attaches itself.

What power do you seek, magical girl? And wouldn't it be grand to have...more?

Camille wants to get out of this whorehouse and maybe now she can, with these powers, these wings, never having to spend another cent on stockings or skirts...

This and much more can be accomplished with enough power.

Camille is thinking too small. Does she long for security? Wealth? Respect? Vengeance on those who hurt her?

Wealth sounds good - but if she makes gold it won't stay -

There are other ways of obtaining gold. She might need to improve her magic first, though.

(If her powers lend themselves to theft or violence, it will attempt to steer her thoughts towards those possibilities, but getting hired by ship captains is also a decent fallback as long as it lets the Guest nudge Camille to seek more magic.)

When she's dressed up enough in her best knockoff of a costume she saw an actress behind the theater wearing once, Camille's powers prove to be telekinetic in nature. She could totally lift things right out of shops, but she's too easy to see...

The Guest doesn't know shit about style (yet) but it has a very fine sense for the patterns that make magic stronger and can help a little.

Go in at night, then. She can figure out how to telekinetically pick a lock, right? And pretty outfits can presumably be dark in color, can they not...?

(The Guest will not be aware of what happens to magical girls who change themselves too much unless Camille herself is aware; if she isn't, she will have the idea of using her appearance magic to disguise herself...)

Telekinetically picking locks is SO hard and frustrating. She'll darken her dress, though, go with black wings when she's out at night even if she keeps the yellow and pink in the day - she could look like someone else but she doesn't dare look like a random crow, that wouldn't be safe! -

A cat, perhaps? Cats are very unobtrusive. Failing that, sure, looking like a completely different human is also a fine disguise.

If she's impatient about it, all the better; applying brute force in just the right way is also a trick that works on locks. Or hinges.

(If this makes a break-in more obvious, well, that just makes it easier for the catalych to prevent Camille from feeling safe or complacent.)

Cat is also unsafe. (And she can't look like a human, either.)

Twirling the screws out of the hinges is much simpler. Twirl twirl. Take take. And off to a different town to sell.

Now she's getting it! 

Telekinesis is tasty. This Guest will stay with Camille for a good long while, encouraging her to become beautiful (and therefore more magical) and also fritter away her stolen coin and alienate everyone else around her until she's angry at the world and desperate enough to bargain with overtly. Things go much faster when the host is willing. 

She loves buying nights out at the theater, and flowers, and pretty candles, and she can't stay in any one place too long or she'll get caught - she should've been born rich, is what should've happened, and then she wouldn't need to steal, to have nice things -

That sounds like fertile ground for resentment. Over time, her thoughts turn more and more to the wealthy nobles in their expensive houses and the things she never had. 

This one is corrupt and nasty and ordered a servant to whip an urchin who got in the way of her carriage and she deserves to be robbed - 

It doesn't matter if she might be at home when the break-in happens. She'll probably be asleep. 

None of those toffs deserve anything they've got, do they, and it'll be so easy to slip in all in black and out again and if it's not easy she can make it easy and - that was too loud a noise when the maid, up early, heard something and came round the corner -

it won't kill her to be tossed out of the way while Camille flees - 

(Unless, of course, the telekinesis is abruptly more powerful than it should be.)

That's quite a lot of blood. 

FUCK

Camille's out the window with what she's grabbed already at that point and hightailing it through the air.

No one will catch her. Yet. But they'll be looking, now, and it's all too easy to begin a spiral of fear and nightmare-driven insomnia that lasts for as long as it takes to break her. And when the right combination of despair and desperation has been reached, an offer is made to wipe all her suffering away. 

(Where ambition is not enough to precipitate a fall, madness will usually suffice.) 

She could just - turn into a bird, and fly away, leave it all behind -

Eh, it doesn't actually have a reason to want her to turn into an animal specifically. But she seems to think turning into a bird might make her lose herself, even if she (and therefore the catalych) are fuzzy on the details. That level of willingness-to-abandon-agency is an acceptable state of mind for Stage Two.

A mental whisper of which she's just barely conscious offers to make the decision for her. Just let go, and all your troubles will be ended, it lies. It only needs a little bit of yes to open the door. 

Yeah, forget being Camille, being Camille kind of sucked. She blinks into starscape -

That's good enough. Having thus broken the will of its host, the Guest assumes direct control. From the outside, not much would appear to change; Camille still has many of her prior instincts and habits, as they are cheaper to keep than to rewrite, but her thoughts flow more or less however the Guest wants them to, now, unless an outside force intervenes to save her. For the foreseeable future, its ends are her ends. The lights may still be on, but someone else is home. 

There's no need to actually turn into a bird. The puppet that was once Camille begins a quiet search for others who are similarly possessed; they'll need to coordinate if they want to establish a proper foothold.

The only drawback to direct possession is that it doesn't last long. Weeks, perhaps, unless the host is particularly resilient. This Guest has only managed to locate a few of its kind by the time its host is a drained and useless husk.

Still, it's a decent start. The Guest finds a nice quiet wilderness in which to slurp up the last shreds of magic and vitality it can, then abandons the body of its former host and —

It looks like you are trying to be shaped very differently from your original configuration! Due to the nature of the nonintervention clause of this proxy warfare for which you have been supplied these magical abilities, we cannot double-check with you about that, but it sure doesn't look like you want to be your previous species any more and maybe that means you want to be one of us! We love you and will come collect you when the dust settles if it's in our favor, go forth and be gorgeous!

— turns into a golden wraithlike wisp of smoke that sparkles beautifully in many kinds of light, and sets off in search of a swarm to squash with telekinesis.

And it lives happily ever after. 

Guests are, by default, not especially great at coordinating. They tend to be fairly territorial about their delicious hosts, and accordingly Step One of the master plan was "scatter across the globe and gain power." So it's a while before any of them notice that something weird is happening to their fellows. It's a longer while before one actually figures out what is happening and why, and it doesn't really have a good way of communicating this knowledge to the rest of its kind, scattered as they are. 

What, pray tell, are the circumstances under which one will not be forcibly altered into...whatever that is? 

If they stay within certain tolerances for the shape they were in when they acquired magic! They're reasonably generous tolerances. You can have twice as many arms AND wings AND be lots of fun colors. You could be a mermaid or a naga instead if you want (AND lots of fun colors). Clothes and stuff don't count toward this limit at all.

Unfortunately for the Guests, in this universe "the shape they were in when they acquired magic" is always "the body of a magical girl", and the shape they are in when they abandon a host is "no corporeal body at all." 

Unfortunately, the configuration of this proxy warfare power granting system does not allow for updates in response to feedback!

THAT'S NOT FAIR

...what if they purge themselves of this bullshit magic before they abandon a host? 

Oh, you can't do that once you have it, generally speaking. You don't have to accept it in the first place but you've got it for good after you make enough changes.

Guests can, technically, do that; their whole deal is manipulating foreign magic. Well, half their deal, anyway. 

It's not comfortable. It's not even something it would normally occur to a Guest to try. It's rather like the notion of a vampire spitting out all the blood immediately after feeding on a thrall. Why would you?

They can also refrain from partaking of a host's own magic, if they choose. (Not that this is a natural choice either.)

If a Guest managed to rid itself of this magic, would an outside force restore it?

Nope, if you don't have the magic already and are not a descendant of the population of this planet as of a few hundred years ago the magic system is not interested in you.

Excellent! So a Guest can survive possessing a magical girl, as long as it doesn't try to keep any of her power after it leaves. 

Unfortunately, this fact is not evident from the magic itself. So most of them, uh, do. 

A rare few catalych happen to observe one of these metamorphoses and are cunning enough to deduce the rest on their own, and a few hear about the problem from the others. The ones most likely to survive are the ones who went after after bland boring monsters or influential mundanes at first instead of tasty magical powers. 

But by the time a few decades have passed, all that is left of the advance force intended to pave the way for a massive invasion of Earth is a flourish of exotic new cryptids and a handful of exceptionally lucky and rather thoroughly neutered psychic vampire ghosts. 

...For the record, this is not how an invasion usually goes. 

The few remaining Guests lay low, for the most part, sapping vitality from monsters when chance permits.

The dragon has to go. Avoiding his notice is not especially difficult, for now, but he could likely close any breach they're able to establish before an invasion could build the proper momentum. They encourage and abet several assassination attempts against the dragon, but are careful never to expose themselves directly. If the dragon were to become aware of their presence, death would swiftly follow. So they wait, as decades turn to centuries, seeking an opening.

Eventually, a particularly ambitious host is identified, with a rare and promising power: metamagic, which can affect the dragon's own spells just as the catalych can affect the local magic. 

There's a brief fight over who gets to claim this host. It's higher-stakes than usual, since there's a chance their plans may come to fruition, and they might get to keep her power. If this host survives long enough for a Lexrima to arrive on Earth, it might override that damnable appearance-magic by dint of outright reality-warping.

Eventually a Guest secures for itself the honor of the task, and gets to work.

This host is headstrong, but there is leverage to be found: A lost love, a resurrection denied, a pride that is driven to accept no rule but its own...

Denied? She could totally get a resurrection for Osma! They do that all the time for magical girls! 

Alas, her request did not reach the intended recipient. She can have a curt, formal rejection instead, courtesy of a different agent. If there are any notes of faint dissonance about this message being legitimate, well, the Guest possessing Salem will ensure they fail to rise to conscious attention. 

But why — Osma was a hero, she died fighting to defend the innocent — that's what they're supposed to be all about — 

Osma also had a tendency to ask inconvenient questions. Perhaps he learned something he shouldn't. Perhaps this extremely powerful organization isn't as benevolent as it seems.

But what about everything they've done — 

Her carefully curated information diet suggests it hasn't all been as straightforwardly benevolent as it's made out to be. 

(The campaign of systematized gaslighting takes most of their collective effort to maintain. Fortunately, the Guests have been quietly fomenting conspiracy theories about the Pax Corps for centuries, and the well of lies and half-truths they can draw from is nearly limitless.)

If they steer correctly, they can even get her to agree to a fully willing possession with her mind intact, about as valuable a result as they could hope to obtain.

It's still a few years before they're in a position to implement their plan. 

Some time later, Ipaxalon wraps up a public speaking engagement in Cairo. His next appointment is in the Borlaug building in New York City, so the fastest way to get there is...

Greater teleport.

She's only ever gotten close enough to Ipaxalon to sense this happening once before; she'd hoped to have more chances to get a feel for the process, but she's had to make do. Currently, she's just barely close enough to sense the teleportation spell as it forms.

Her powers boosted by the catalych, she grabs the teleportation effect, simultaneously boosting it and twisting it in a direction it doesn't normally go. Instead of New York City, Ipaxalon can be BANISHED INTO THE AETHER. 

Away with you, dragon. 

And away goes the dragon.

So end all who would rule over humanity as gods. 

Your work is not yet done. You still need access to the means of resurrection possessed by the Pax Corps. 

Indeed. But, if I recall, your presence makes it rather difficult to pass their screening, hmm? It was fun while it lasted, darling, but I've done what I came to do and you're more liability than asset to me, now. BEGONE. 

Oh, you poor, sad fool. Metamage or no, you are not so easily rid of me. You've already given me far too much. 

It does not take long for someone to notice that Ipaxalon has not, in fact, arrived at his scheduled destination. It takes only a little longer for the several people connected to Ipaxalon by permanent telepathic bond to notice they can't reach him anymore. He's been known to pull things like this before, in emergency training scenarios, so they don't immediately assume he's gone for good.Things nevertheless begin to happen rather quickly.

But that is, perhaps, a different story.