It's well-known that some places are haunted, spectral. Every town has at least one, an old building – often a home – which just sends the shivers up a pony's back, from dock to poll, raising every hair from tail to forelock. The windows seem to stare inwards at rooms' inhabitants; darkened doorways gape like an animal gasping for breath; shadows lurk at the bottom of stairwells and in neglected corners. Often, there are no actual ghosts that any pony can find. Sometimes, there aren't even any stories to accompany the haunting sense that something is watching. Not, that is, until pony imagination takes its course, and suitable narratives are manufactured by the clever for the edification of the credulous.

It was a great surprise to everypony when such a haunting began within Ponyville's newest and gaudiest dwelling, the Castle of Friendship.

It had seemed like a good idea at the time to those responsible, to bring the severed roots of the princess's former tree-home into the new castle. There are no real procedures for burying the corpse of a tree after its death, let alone a tree-library like the late Golden Oak Library. One of the few fatalities of the brief Second Centaur War, its loss had been strongly felt by its former inhabitants and the denizens of the community which it had served for a century and a half.

So when the former Elements of Harmony dug up the Library's dead roots, and hung them in the throne room of the new Castle of Friendship, nopony really thought it macabre or in poor taste. It was difficult for ponies to equinize something like lumber, to think of those roots as what they were. It was, in their eyes, simply – well, rustic. Charming.

A reminder of what had been. And a way of easing the lonely heart of the Castle's princess, who had bonded quite strongly with her former home and place of employment in the brief span of months she had spent with that homey, warm dwelling. The Golden Oak Library had housed generation after generation of pony librarians in its period of existence, growing around their lives, cupping them in its boughs, keeping them warm in winter, shaded in summer, and sheltered against the storms of spring and fall.

The old library-tree must have come to recognize the feel of new books on its wood-grain, because during its later years it had begun to bud new shelving even before a given librarian could think to prepare their new arrivals or purchases. Spike, the princess's number one assistant, had always wondered why the tree-library had been so devoid of insects and other pests, why he'd never found any worm-bores or dryrot or silverfish. Then Applejack pulled some books off of one of the lower shelves, and showed him the little growths along the back of each nook, little growths which the farmer assured him were producing a clever cocktail of chemicals designed to repel insects and fungal growth.

A good library-tree protects her books.

So, perhaps, it was not so strange for a librarian-princess to keep the increasingly desiccated remains of such a devoted defender of books in the heart of her cold, sterile castle, which she intended to fill with as many books as Equestria's struggling publishing industry could provide. Those roots, overhead, made the chilly crystalline walls just a little warmer, made the castle's polished flooring feel a bit less slick and treacherous, deadened the glaring shine of its lighting into something a bit more gentle, and more like a glow.

All well and good, and what the Princess's fellow former-Elements had intended by their gift, at least in part. If only it had extended that far, and no further!

It was the accumulation of new books that triggered the further changes, investigators later concluded. Ponies don't generally put much stock in talk of afterlives, the spirits, and what happens after a sapient being ceases to be. Insofar as they do give any thought to the subject, they tend to import the superstitions of the griffons, the zebra, and the minotaurs, rather than generate their own wild imaginings. One can search the books of fancy, the records of sociological studies on the superstitions of those three nations, for a very long time, and never find speculation about how the spirit of a place, the anima locus, might somehow become untethered, and haunt another locale at some distance from the original place-that-was.

And yet…

The first sign of the awakening of something which really should have been dead, was probably missed by any sapient being, but the first detected sign, was when Spike the Dragon was shelving some new arrivals in the small but expanding section of the Castle which the princess had designated 'the library', and turned around to find his just-shelved books missing from the cold, slick surfaces the castle had obligingly extruded as 'bookshelves'.

Now, nopony actually liked the cold bookshelves the Castle had extruded; nopony really relished the idea of a building which extruded anything at all, let alone shelves. But the Castle was the creation of Friendship, of pure magic, and everypony involved felt the need to pretend that this chill, sterile monstrosity was something wonderful and delightful, no matter how much it gave everypony – and dragon! – the willies.

Spike honestly thought at first that it was the Castle itself which was playing games with the new books, and reported it as a bug to his princess. It wasn't the first they'd noticed – the less said about the unfortunate early issues they'd encountered with the plumbing and water-pipes, the better. But the princess had discovered that the Castle could be persuaded to shift itself with the right stimulation, to re-grow pipes in the proper configuration, to remove walls that shouldn't be there, to grow them where they should be, to form doors and windows as desired.

Which didn't mean that the Castle was alive – not like a tree was alive. Crystals grow, and decay, and flow, but none of that meant that crystals were life. Whatever you might think about crystal – pretty, organized, intricate – it did not have spirit or energy. Crystal's entire value as a substance was that it had none. Crystal had no existence, it was simply… a vessel. It held, very well, very securely. It was why crystal was so useful in magic, in enchantments and other spells that required a reservoir or a container to hold a process until it was required for later stages of whatever elaborate ritual you were performing.

It was probably this quality of the crystalline Castle, which made the haunting possible – even inevitable, once those roots had been hung in the throne-room.

Spike's quest for the missing books had no quick or happy conclusion. He simply could not find them, anywhere within the confines of the Castle. He even had the princess check for lost hollows, interior rooms which might have been accidentally walled off by the Castle's occasional natural shifting, as the crystalline construction re-balanced itself. The original form the Castle had taken had been badly, dynamically unbalanced, and much of its first three months of not-quite-existence had been punctuated by occasional lurching shifts as it laid down new structural elements to keep itself from pitching over in a heavy breeze.

The books were never found, and worse, others began to disappear from already-populated shelves. Even the princess started noticing the losses, and she locked the doors of the Castle in a brief burst of paranoia, convinced that pranksters or bibliophilic thieves were to blame.

But no thief was ever found, and not even Rainbow Dash could be blamed for the disappearances – for some reason, no A.K. Yearling edition was among the lost books.

It was when the princess was sitting in her throne room, in her throne, staring melancholically up at the dead roots of her lost home, that events finally came to a head. The story goes, that she stood up from her meditative recline upon her throne, and walked over to the Prench doors she had installed between her throne-room and the designated library beside it, and went through those doors while looking over her shoulder at the roots, rather than paying attention to where she was walking.

And when she had turned around to find herself in her library – dark, as was appropriate at that hour – but somehow… nostalgic. She found her way to her shelves more by scent than sight, not bothering to light up her horn. The princess was, perhaps, half-asleep in this moment, and not entirely paying attention to her surroundings.

Which, no doubt, is why she didn't notice the wood grain of the flooring below her, or the living wood of the low ceiling above. But what she did notice, was the shelves full of her missing books, the books they'd torn the Castle to pieces looking for – all carefully sorted, and shelved, and cataloged as she had always preferred them.

On living oak book-shelves.

From that day forward, one could walk through every crystalline corridor of the Castle, and if you were determined to find exactly that, this was what you found. Nothing but miles and miles of crystal, perhaps more gently lit than should have been possible given the harshness of the castle's natural lighting and the given time of day, but still – as standard and common a castle corridor can be, that happens to be made of thaumically dynamic crystal.

But if one was in a nostalgic mood, or were feeling the need for a gentler, less confrontational existence, then the hard, somewhat chill chambers and corridors of the Castle can and will became something else, something more worthy of a 'Castle of Friendship'. Sometimes this was only a feeling, an experience, and if one looked with skeptical eyes, one would only see the common, expected sight of crystal and decoration and the like.

But if one relaxes one's expectations, and stands waiting, unexpectant, empty-hearted… The Castle's Heart might just open to you, like the blossoms of a fruiting tree. Living walls of oak, shelves full of well-loved books, and over everything, the soft glow of a library grown for readers and lovers of books.

And that was that, and that was fine, and the Castle became a site of pilgrimage for scholars, for writers, and authors, and students. And that would have been all, if the haunting had ended there, a simple, happy bit of mystery in the heart of the realm. And so it was – for a while.

Until the first dying pony walked into the Princess's foyer, and limped through the oaken heart of her strange Castle. When that sickly pony disappeared somewhere in the stacks, and was never seen again, her grieving family spent weeks exploring the ever-changing corridors of the distraught Princess's home. But nopony ever saw that pony again, and the rumors started to spread...

***

They say that when a pony's body is failing, and the world grows hard, and dark, and silent, and full of pain, that there is a place one can go. When you're tired, when you're ready to move on... So my grandmother told me, her eyes blind, her ears so deaf that she could not hear my pleas to stay, her hooves so dead to sensation and feeling that she couldn't even stand with her walker without assistance.

And I, as any good colt would do, helped my grandmother to her legs, and walked slowly beside the contraption which was the only other thing that allowed her to move under her own power, such as it was. We slowly walked through the harsh summer afternoon light, through the rich lanes of proud Ponyville, the summer capital of all Equestria, ponies all around us enjoying the wealth and prosperity provided by the rule of the justly famed Dusk Princess, known across worlds and universes for her wisdom and kindness.

Eventually, we came to that princess's castle-gate.

In those days, the Dusk Guard posted the gate, as suited the dignity of the home of the great sovereign. There was no ramp before the public portal, but rather a series of steps, challenging to the elderly and their helpers. Not steep, mind you, but a significant barrier to a pony beholden to a walker.

The Princess did not care to encourage visitors such as my grandmother.

We made do, and I did my best to leverage my deaf, suffering, blind old grand-mare up that set of stairs, and I swear it felt like climbing the Canterhorn. The guards glared at me, silently judgmental, but the Princess had also decreed that, though nopony in the Castle would aid such visitors, neither would they raise a hoof against them.

We passed through the Crystal Gate, and the squeaking of grandma's walker's wheels against the cold floors echoed beneath vaulted ceilings. We walked for what felt like an infinity, but was probably only a hundred yards or so, before I looked down again to see if grandma's legs were giving out again, and I noted that we no longer were on crystalline paving-stones, but rather, living oak flooring.

Well, in a manner of speaking.

As we moved deeper into the Castle, the walls turned from crystal to oak heartwood, and shelving full of ancient books appeared on every side. The titles were odd, and half-familiar, as if I had once read some of them in a dream.

As we moved down a series of curving heartwood passages, the shelves grew more populated, and the quality of the light shifted, grew less harsh, and more dim, warmer, easier on aged eyes. They say that day never truly reaches the heart of the Castle of the Dusk Princess, but one always can see exactly where one is going, no matter how faint the lights become.

Finally, we came to the far side of the Castle, and wide window-panes looked out onto a moonlit landscape, a curving, wooded garden shaded by tall, broad-leaved trees of a species I have never seen elsewhere in Equestria. Some say those trees and their garden lie nowhere in Equestria at all, and that no living pony has ever set foot under their boughs.

I do not know. I did not attempt to enter the garden myself, the one time I saw it, and I have never since found that corridor with its wide windows, and the door that opened out into the moonlit garden, though I have searched the castle's hallways time and again, hoping for another glimpse.

My grandmother's strides lengthened as we passed window-pane after window-pane, as if the years were dropped away from her with every hoof-step. Just before the final door, she handed me her walker, and I looked at a mare I had never seen, except in old sepia-tone photographs, looking solemn and expressionless under the basilisk glare of the photographer's flash. This was that mare, and yet, it was not. The cataracts were gone from her eyes, and her mane was full and beautiful again, and her hide unwrinkled from the lifetime of impossibly broad grins that life had carved deeply into her face.

My grandmother thanked me for my aid, and the last words I said to her was, 'you're welcome'. Then she opened the limnal door, and she walked out into the wooded garden, out under those broad, elysian leaves, and sweet moonlight filled her glowing hoof-steps as the door made of moonlight and oak heart-wood closed behind her.

Ponies say that when you are tired, and running short of life, and the end of your days are coming near, that there is a Castle which is never quite itself, even in the brightest light of sacred Day. But, within its shadowed corridors, its gentle light reveals - in its crystalline setting - a wooden dream of things past. And deep within that crystal fortress's oaken heart, hides a passage into eternal, sweet-leaved night.

Author's Notes:

Thanks for editing and pre-reading help to Oliver, Shrink Laureate and the general Company.

And special thanks to Oliver for helping me with the front-piece art, as it was beyond my extremely limited photoshopping skills.