I Am Vice Principal Luna, and I Will Be Your Swim Class Instructor
My sister and I were fighting side by side, shoulder to shoulder, our horns glowing and shooting beams of scorching energy and our eyes and ears wary every part of our bodies minds and souls alert because we knew we were in terrible danger
or worse than danger, because, yes, in an 'ordinary' war or a fight to rule Equestria a pony can be injured physically or even killed, but Discord can do worse I know he can drive a pony mad, or ALL the ponies mad as I know because I've seen it happen to more than a few ponies, entire villages, even cities, that lived under his attentions or worse yet he simply plunged them into a horror and forgot about them and when he noticed them again it was too late for anycreature to save them and
that's what a tiny part of me still fears most of all, that happening to all my ponies, that happening to me, that happening to all my ponies AND me or to me AND my sister or just to ONE of us as the rest of us look on helplessly and I don't know which would be worse and
sometimes I feel in a way it's already happened to us, because that's what the Mirror Pool does to a pony's duplicate, making her always continually remember the moment she fell in, thinking about whatever she was thinking about exactly then, feeling the feelings she was feeling as she fell in as she plunged beneath the surface
and that's why Canterlot School District has a lot of water safety classes starting at an early age, because a lot of the creatures here, who used to be Mirror Pool clones before they were pushed through a Mirror Portal to here, their deepest memory is also their deepest fear, the fear as they went under the water that they would never ever come back up they would just die and no one cared because whoever had thrown them into the pool or pushed them in would never save them and
do you know what it's like for your strongest thought your most constant thought the one you can never fully get rid of or even push ALL the way to the side to be the feeling that you are going to die and it wasn't an accident somepony or some other creature pushed you in
yes somePONY
yes all of us were ponies before we were HUMANS, before we became these strange towering tottering forms of exile, our shapes as we hide here from an Equestria where those few who know of us prefer to pretend that we are not real were never real, they prefer to pretend there never was somepony getting lost in the woods and trying to find shelter in a cave, or the battle against an enemy (for Celestia and I, that enemy was a Chaos God who tricked us into the cave and KNEW what would happen to us and laughed and laughed and laughed as we struggled and screamed as he pushed us under the water and held us down)
or just as bad in its own way, there was a cult of ponies in a village near ANOTHER of the hidden mirror pools, a cult who believed that if they brought a newborn foal to their 'sacred pool of sacrifice,' and they doubled their foal, they could sacrifice the copy to another world and that would be enough to satisfy the evil gods of disease of famine of war of accident and other misfortune and all the very worse things that can happen to a pony
and their own foals would PROBABLY be spared (according to their misguided, cursed beliefs),
and really, that's how we get some of our happier citizens in this town, the people whose memories of their falling into the water are faded, because it happened when they were newborns or only a few days after they were born, so they didn't quite learn evil exists, not really, and the constant echoes in their head of whatever they were thinking as they struggled for air, well, those foals were so tiny they weren't yet thinking in any words at all, and everyone knows very few ponies or people can remember things from when they were only a few days old, so the poniespeople who were brought to this world that young are almost spared the curse of knowing of remembering and they're not so bad off, not really, feeling unease and fear whenever they're in the water, but having almost no memory of WHY
but even so, those children's parents bathe them in their cribs, or in an EMPTY sink, wiping them down with a cloth or a sponge, making sure they don't feel the fear the terror of being dunked into the water, of water surrounding you, fear of going under AGAIN, fear of struggling and fighting until finally you gasp and your body tries to breathe water but of course it doesn't work, nothing will do any good there's no rescue you'll never quite have your whole self back again because maybe all you can do is fear fear fear or anger anger anger or hate hate it wants to kill me I want to kill it first how can my mother my father the other ponies do this to me and
I mean, the ponies-children who came here youngest don't have so much of that
but water safety education is a very slow and careful process indeed,
learning not to panic
learning not to drown
learning not to scream
learning that the water can be ok
even while somepony I mean someone is swimming they can still learn to think about more than one thing at once do more than one thing at once even then even then
and that's what I mean
and that's why being human Luna, copy Luna, clone Luna, Mirror Luna, who you also know as Vice Principal Luna, teaching students how to conquer your nightmares of water, is so important
because SOMEONE has to make sure that for each and every foalchild who lives in Canterville, who attends our school,
water might not be a friend, but at least if it's an enemy, it's an enemy we can learn to conquer, to beat, to at least keep it from making us panic and the panic itself making us slide under the water again oh no no no no how can this happen to me again
even though (the terrified ponyperson thinks and feels) I always knew the water would happen to me again, because I always was every second thinking about it ever since I first became me, since I first became the mirror me, the copy me, the me me that in the beginning wasn't the real me but now somehow I'm as real a me as anyone else
because I was sure the water would happen again but somehow I learned: slowly slowly slowly oh so many tiny steps to learning, how to wade how to float how to swim even how to dive
and the thing that I knew would happen
when the water surrounded me again, I made happen differently
not just a nightmare, but also a dream of conquering that wet watery enemy, maybe even some safety in the water, maybe even someday a kind of healthy contentment, and
yes, someday all that can be yours, being in the water and not needing to be afraid the horrible thing will happen to you again
so welcome, each and every one of you
to Canterlot High School Swimming Class 101.
Author's Note
A few days ago, RDT remarked:
1000 word easily understandable and grammatically correct sentence
Is such a thing even possible?
[...]
You'd need a repeatable structure that was human-understandable and followed grammar rules
[...]
Your eyes may die, but that's a sacrifice I'm willing to make.
But there's some things that let you repeat clauses
At the same time that (full sentence), (full sentence), (full sentence).... and (full sentence), (full sentence).
I don't think this is really what RDT envisioned. 
I omitted more than a few commas, for flavor purposes.
I suppose one person would be interested by the fact that there might be an (unpunctuated) sentence boundary (or just a colon omitted to change the text's flavor) partway through (but near the story's beginning) at
but Discord can do worse I know he can drive a pony mad
