Canterlot... For centuries, Canterlot was the shining gem of Equestria, the pinnacle of pony society. Now, it’s a reminder, a bleak and monolithic warning to the world. 

Changed by the chemical and technological poisons that plague the mountain, an abomination of nature, I wander the greatest disaster Equestria has ever seen. 

... I will escape eventually, and when I do, I hope my friends still see me as Twilight Sparkle, and not the monster that hides among the ruins. 

Daring Do cannot believe her luck when she is asked to help explore the most ancient tomb known to ponykind. But terrible danger awaits her, for beneath the earth rests something beyond equine understanding.

Warning: comments contain ending spoilers.

TVTropes page.
Alternate ending by Duplex Fields.
Archived in the Pony Fiction Vault.
Reviews:
One Man's Pony Ramblings
Louder Yay
PresentPerfect
The Royal Guard
Cleverpun's Critique Corner
The Goodfic Bin

In her quest to eliminate all chaos, the one now known as the Empress of Harmony established her rule. With the essence of her friends removed, only those she deems pure are permitted to live under her watchful care.

Order and Chaos are divided, those who support Twilight in her noble goal versus those who oppose her.

Harmony is the greatest good, as the Empress Twilight Sparkle insisted. But can Harmony really exist if the bearer of Magic has become disillusioned between justice and vengeance?

(While this is a sequel to Pages of Harmony, I have written it so it can be read as a stand-alone.)

Starlight Glimmer awaits her punishment at the hooves of Twilight Sparkle and her friends.

After the changelings' failed invasion of Canterlot, Twilight tries to soothe some of Ponyville's rampant paranoia by casting a spell to prove there are no imposters hiding in the town. But her spell leaves her in an alien body, facing a mob of angry ponies who just don't believe her when she says she is Twilight Sparkle.

Or, at least, she was Twilight Sparkle, anyway...

Featured on Equestria Daily, 2/17/13
Cover art is by the awesome and amazing Conicer

When Twilight Sparkle slipped beneath the covers last night, everything had been fine. She had friends whom she loved, a teacher she adored, and a bright future ahead of her. But when she woke up, her blankets and quilts had been replaced by hospital gowns and padded restraints.

Everything is wrong and nothing makes sense; even her friends have changed. The doctors keep telling her that she is sick, that none of it was real and that it was all in her head. Yet she remembers a life far beyond the hospital walls. She couldn't have made it all up on her own. They must be lying… right?

Ask Asylum Twilight - Still updating regularly!
Equestria Daily
TVTropes
Asylum Fan Group
The Tartarus Project
Russian Translation
The Asylum 20,000 View Spectacular Stage Show!

Art:
Asylum Cover by conicer
Twilight and Twilight Smiles by dreamingnoctis
Mirror Twilight and Plague Doctor by Jaestring / BloodGoldWings
Twilight Smiles and Doctor AJ by conicer
Nurse Ratchet and Silas and What's in the mirror? by Archonix
Doctor AJ Sketch by Archonix | Colored and vectored version by ReFro82
Twilight's Asylum by Khan the Cake Lover
Smilight by HappySwitch
Twilight Waking Up by Dombrus
Fan Art by toxicdemon10
Asylum Twilight by Madness-With-Reason
Asylum Fluttershy and Asylum Ch 26 by Downburst-Backspace
Twilight and her Twin by v747
Birdgirl by sirValter

Readings and Videos:
Asylum Animated Trailer Video by formuladash
Reading by Gammarobot (SugarCubeRadio): All Chapters Here
Reading by Ender Brony: All Chapters Here
Reading by VisualPony: All Chapters Here
Reading by MelancholyIguana: All Chapters Here
Fan Trailer by Kaidan: Asylum FanFic Trailer Cuckoo's Nest

Sanctuary by Kaidan
Broadhoof Files: Dr. Humors by TypewriterError
Broadhoof Files: Corporal Phalanx Spear (Ret.) by Phalanx Spear
A Hearth's Warming Tragedy by Seven Fates

A special thanks to Breath of Plagues, Rex Ivan,
ChudoJogurt, Cynewulf, Death the Kid, Selbi, PrinceDolph, Blahman2816, Queue, and Gage!

Axan Zenith - Dr. Dreamer
Dratini4 - Way Finder
psychicscubadiver - Bright Mind
Velkaden - Dream Chaser
Reese - Silver Glow
Viola Heartstrings - Clover
Archonix - Spinney Whiteacre