Demyx is pretty new to this.
Of course, he's only existed for, what. Must be more or less a moon, now, though time is... strange, here, in this world that doesn't actually exist anyway, or so they say. But more or less a moon, in any case.
So yeah, he's pretty new to this, and so when III - whose name he hasn't been told yet but carries the wind around with him (and Demyx knows the wind, has always known the wind, and the wind may be his ally but he can still swallow it whole, if he wishes, he can, trap it in a cute harmless little bubble that dances on his command), which is almost introduction enough - hands him a card and says go, in a tone that brooks no disobedience, well, what can he do? He enters the Dark Corridor that's suddenly there, and walks back into reality.

--

He's on top of a mountain, in a world he's never been to before. It's raining, down below, not water-onto-rock and water-into-water, with the wind howling and ripping things up, but just a soft, steady rain. He's not quite sure how he knows this, above the clouds as he is, but he's pretty new to this. He hasn't quite figured out which bits of him are him, and which bits of him are water, and the more he tries to think about it, the less it seems to matter.
He reads the card. He stares at it, a little, and then reads it again.
Are all of those collections of letters really words?
Apparently so, because it does in no way seem like jest, and, well. He frequently gets lost when The Superior starts talking, and it's apparently from him.
So Demyx looks down and translates what he can. It's not much, but it seems to say we need heartless to get hearts, and to get heartless, people need to die, so what he's been asked to do is pretty simple, really.
Kill everyone.

So, he's on top of a mountain, which he's now sure is the tallest mountain in the world (and it's not even cold up here, it must be a pretty flat world, all in all), and there's people down below. Whom he needs to kill.
Demyx doesn't like fighting, but even if he did - what good would it do? There're thousands of people, down there, and Demyx knows folklore well. He's the villain, here, and if he appears before the people, a hero will appear and vanquish him. Fighting will do him no good at all.
But Demyx is water, and he knows how to fit into things and still get the job done.

Down below, a good listener may have heard a soft plink-plink-plink, like someone tuning an instrument. But even if they did, they ignored it, in the bustle of the city day, as just another bit of noise pollution.

The rain has passed, naturally, and it makes Demyx sad, a little. He likes rain. He's still thinking about it, when he plays an arpeggio (IV had told him he doesn't need to warm up, now, that his weapon is part of him, that it will never be out of tune - but it's an old habit, this one, much longer than his existence, and will take a good long while to break), and there is rain, again, falling softly down his neck. Demyx smiles.
He lets the rain go on its way, and plays.

He starts, of course, with a prelude. The sun shines down on the flat little world, and the water sparkles, stretches. Wakes up, for Demyx, and is Demyx, because Demyx is water and water is Demyx. They-he have work to do.

Children play on the beach, the world unaware that anything has changed.

Softly, carefully, he plays a minuet, the one-two-three, one-two-three a perfect start, for what he's doing. Come to me, come to me, he plays, and, awake now, the water begins to rise, swallowing the mangrove forest by the shore.

It's hours, before the residents notice, and by then it's too late.

The people-prey-humans-targets on the ground finally see his water coming towards him, but they're... not really concerned. The tide, he's sure they're thinking. Just a big tide. We'll get a bit wet, but it'll go down soon.

It is a tide, thinks Demyx. A really big tide, even. Biggest tide you'll ever see, for sure. I'm closer than the moon, and water likes me better, anyway.

He shifts gradually into a bolero, as the sun sets, turning the sea red, like blood, like fire, like a million other things that he's never given a damn about, even when he did have a heart.

The people don't start panicking until after the sun goes down.

There's screaming, down below. There's not all that much ground left, now, and he's not been violent, not at all, so there's a lot of survivors, at this point. Before, he would have become part of that madness, stirred the crowds to riot, to come apart at the seams, and he would have loved it, would have felt the biggest rush-

Now, he sits on his mountaintop (little more than just a hill, now), and keeps playing his serenade. The water swallows all.

Eventually, there's nowhere left for the people to go.

Demyx looks down at them, close enough now that he could count them, if he wanted (he doesn't, and they're in such a panic the number keeps dwindling, anyway), and thinks. Too much further, and they'll reach him, and that wouldn't be good. That wouldn't be good at all.

He plays a nocturne, and the dark, dark waters rise up for him. Time to hunt, he thinks. Nothing else left for him to do.

None of them ever reach his mountaintop, but Demyx covers it anyway. He plays a requiem, softly, for the soul of the dead, dead world. Water, everywhere the eye can see. He's in it ankle-deep, but he's not wet at all.
But the thing about a dead world is there's no one else to talk to, and so he summons a Dark Corridor, to take him back to non-existence, where he'll at least have company.

--

II is waiting for him, when he gets back.
Demyx kind of likes II, he's not a blank piece of nothing like the rest of them (and no wonder they think the situation's hopeless, when they all act like that!), and he's not over the top, obviously faking it, like Axel does. Axel, Demyx thinks, is really kind of weird.
II (and he's not been introduced to II, either, and though he always feels a little bit contained when he's around, he doesn't think that's a bad thing. Water evaporates, if you spread it out too far) smirks at him, and says, report in, kid, and so he does, and then says what?, when he's finished, and notices II staring.
II just laughs at him, long and loud.

Demyx is pretty new to this, and he thinks he might have just fucked up.