It starts on the horizon, creeping over the land. The storm clouds are huge. They fill the sky. They are moving this way; relentless like the tide, faster than they have a right to. They are dark, so dark to be nearly purple. The rain begins to fall near you. You hear it hit the ground. It hits like bullets. The wind gusts; getting in between a person and their warmth. There is thunder that deafens you. It rolls forth like a stampede. The lightning brings the suns brightness for a moment, before plunging you back into the dark.
It is a storm. It is nearly a storm front. It has no anthropomorphic eyes or mouth. It is composed the elements of storm - wind for a body, lightning for thoughts, thunder for roars, and ice and water for its teeth and claws. It is a storm that moves and shapes itself of its own accord, even against prevailing wind (though slowly). For all its oddness, it is still recognizably a storm, it mostly acts like a storm, yet it is not just a combination of heat, cold, water, and air.
One must look at it mystically (or with a Clarkian Tech) to truly "see" it. The storm has more energy than just physical energy. It has, in its center, a bundle of organized energy - a matrix - more concentrated than the energy distributed in its "field". There are flows and astrone whirls.
If you need a more material analogy; the matrix acts like a nucleous, the mystic energy as the cell liquid/ cytoplasm, the whirls as ribsomes, plasmids, mitochondria, and other parts of a "cell", the outer shell of the storm, where it meets the free air, is its cell wall.
Keep in mind that The Storm has mystical senses. It sort of "scans" an area for an energy type. This allows it to sense weather fronts, pressure zones, temperatures, and other weather phenomena. It also allows it to sense general magic. It is hard pressed to find specific details, but it knows a general area. Like you would be able to pick out a small pebble somewhere near your feet easily… think about it’s scale. People are really tiny to it, kind of like trying to get at Ants.
This energy being is from "somewhere else"; it is not native to the realm. How it came here, it is hard to say with certainty. It could of fallen through a rift in space time. It might of pushed through a soft spot in realities. It might of been summoned, either dragged against its will or early awaiting release to the world. There is a chance that it evolved naturally, spun up from some combination of mystic energy, will, and emotion - manifested into the sky. While its true origin may be open to debate. Its existence is not. It exists.
You will need to choose one that fits your campaign from the options I have given or make one of your own.
Upon being in our world, it extended itself - harmonically - to the material it is comfortable with. We call it air and all that makes it up. Any organized mass of air and energy becomes recognizable as a storm.
And quite a storm it is.
Like all creatures it has motivation and drives.
Hunger is one. It needs to absorb ambient mystic energies to properly empower itself. If it can find a magical region, it would stay there "grazing" for a time.
It can convert "weather energies" into sustaining energy. It gains power as the heat/ pressure/ moisture moves through it. So if it reaches a warm front, this would increase its "storm potential" and its overall energy, without it having to resort to expending its mystic energy.
It is a simple intelligence, an elemental one. It is more than an animal, but less than an adult. It can plan and predict actions to a limited degree, but seems to have attention span and follow through issues. So think of it as the world’s most powerful cranky 4 year old. Adjust intelligence to taste of course.
The Storm might be seeking something. It could be a Geas from someone who consciously summoned it. It could be what summoned it in the first place (or could banish it). The players might have moved through a rift recently, thus stink of it. They might seem like food, or seem like a way home, or just be seen as the people who brought it here? Has someone read one of those Lovecraftian books recently? They might be this worlds counterpart to the Mighty Being that that threw the Storm into the void. It could be following and trying to get an item of magical power so it can feed enough to reproduce. It might be tracking the most powerful magic in the area just to feed or might be worried that it would hurt The Storm. It could be looking to avenge itself against someone for some reason. It will have a reason to be acting out, though the players might not ever figure out why it is hunting someone/ something. This is the crux of getting it into your campaign.
Effects will start mild and grow in intensity. The attacks will seem directed after a while. Sometimes it will take its fury on others, nabbing up innocents who strayed too close in the meantime. The key to making this work is to play it in the middle. Some of it seems directed at the PCs (or target of The Storm’s Wrath) and the rest seems to be the result of a random bad weather chart.
How to deal with The Storm
We all know players, they are going to want to try and fight this thing, instead of thinking their way around it.
If you have access to big weather magics, you can combat it directly. Big If. It can also throw more lightning than most mages can. If you can dump enough cloud seeding material into it or cause it to ground itself (cloud, mountain, big lightning rod…), it can be harmed.
Ideally, you are going to find a way to satisfy The Storm’s motivation/ need. You will need to deduce what it wants or need. Hopefully you will either be able to give it what it wants (or a substitution) in a way that your characters can accept.
The other ways is to weaken it. You must force it to over extended itself in some way. It will often "follow its need". It can be led into areas that will give it issues. You can lead it into another high pressure zone. You can head it into an area of cool temperatures. You can head for a very dry zone. You can lead it into a low mana zone. These changes will cause The Storm to extend itself and its resources to pursue its goal. It might have a moment of smarts and break off pursuit. You might have to shake the bait, to keep drawing it towards a danger area. It might just wear itself down to get what it wants, not realizing what it is doing to itself until it is too late.
Roleplaying continued and Storm Biology
If it encounters another storm, it will have moments of issues. The initial conflict will be like two animals fighting each other. It will be chaos and damaging. The Storm will win, as it is "smarter" and more aggressive than your average storm. It will then spend a few hours "eating it". Taking in its moisture, its heat energy, and electrical charges, The Storm will grow.
The Storm is a creature of rage and drive, disoriented by its manifestation. Like a child, it is not thinking about why it is doing what it is doing, it just is doing it. Given some time, it might be "introspective". The Storm will calm, some. It will remain an organized mass of air. However it changes from an active storm to an area of instability - an area of high moisture, high energy, and electrical change. It will create an overcast or high cloud cover, and be able to shift wind patterns. Note: this will take some time and gradual steps to occur. Given a little time it will also be able to do large shifts in air pressure and manipulate weather activity in a large scale. If it gets agitated, it can change itself back to a full storm in just a few hours.
The Storm wants to be like any normal violent weather front. It does not like to act in unnatural ways. That fatigues it. If it acts along a more natural way it is "easier" and "more natural" for it, much the way walking and talking is against ballet dancing and singing opera. If it extends itself for too long or with too much "exertion" it will become tired and possible calm itself. Or it might just "fade into sleep", like a toddler that just drops asleep. A nap will last for anywhere from a few hours to a few weeks. Of course, it will be cranky when it wakes up.
Note: It communicates via a complex system of charged airflows. However, there is nothing else for it to "talk to". Someone will want to try telepathic contact. This will be hard as its mind is quite alien in to a human, and the reverse is true. It might be quite painful to the human. Some limited and careful contact might be possible. Empathic content might work better, as it has a more common denominator across reality.