"The Princess of Magic."
Twilight's formal title had been bestowed upon her innumerable years ago, when she was formally bound to a patron. Like Celestia to the sun and Luna to the moon, Twilight was intrinsically linked with magic, becoming a part of the concept itself. In a sense, she had become magic.
There is not a single decision that anypony has regretted more.
If only she and Celestia had thought it through. If only they had considered the final outcome. If only she had thought it through.
She should have known, all those years ago, when she asked Celestia what her change meant for her lifespan. She had guessed the answer already - If magic stopped existing, so would she. But she hadn't thought it out, hadn't considered all the implications.
It wasn't the fact that her friends died first. That was one of the most horrible times in her life, but she had gotten through with the help of Celestia. It started with her brother, and ended twelve years later with Fluttershy. She still remembered embracing Cadence, tears hot and wet and hollow like the loss she was feeling for the first time. At the time, she thought that would be the worst feeling she would ever have.
Now, she pleaded to have that feeling again. It would be like pure joy compared to her current life.
Ten years after the last of the elements of harmony was laid to her final resting place, the hollowness started to go away. Twilight felt alive again. Yes, her friends were gone, but she could cherish the memories. Besides, she would see them again someday. Or so she thought.
Twilight went about her life, studying spells and magic. Spike was her constant companion, although he soon got big enough that she only saw him occasionally. While the death of a student or friend was never easy, Twilight managed to cope. The other princesses played no small part in this, and were always there to lend a hoof or a shoulder to cry on. Although she still sometimes missed her old friends, she found her relationships with Luna, Celestia, and Cadence growing until they were just as strong as the bond she had shared with the other elements.
Spike died a few thousand years after Fluttershy, leaving Twilight feeling hollow again for a year or so. With the death of her faithful assistant, her last tie to her pre-princess life vanished. It felt like she had lost a part of herself.
Still, life moved on. Twilight took on her own apprentices in her studies, although none ever achieved anything high enough that she could bestow immortality on them. She never married. The world around her grew and changed as technology improved, thanks in no small part to Twilight's studies. She found herself growing wise, becoming calmer and more articulate. She learned to accept the deaths of mortals, even if she could not entirely remove the pain. Things, for her, were looking up.
Until Celestia's mane turned red.
Nopony expected it to happen. Nopony expected to see the sun slowly creep toward Equestria, Celestia powerless to stop it. Nopony expected to die of dehydration or heatsroke as Celestia's patron slowly expanded into a red giant.
For the immortals, they didn't expect the heat. The burning.
Luna and Cadence were lucky. The moon was quickly disintegrated in the midst of the red giant, taking Luna with it. Cadence managed to live slightly longer, as all three of the remaining beings tried to focus on their friendship to escape the horrible things that were happening to their bodies. But, unable to die and swimming in plasma, one's mind eventually fills with nothing but the thought of agony, leaving no room for love.
Twilight and Celestia still lived. According to her estimates, they lived for a hundred years before they finally escaped. It took all of Twilight's years of training to get them out of the red giant, then far enough away that they could no longer burn. From there, it took even their immortal bodies a few decades to recover to the point where they could talk.
Twilight still remembered those years, encased in a bubble of magic, looking on at the distant star. Just her and Celestia. They mourned the loss of the others, but simultaneously envied them for their fate. They had gotten to go on while Twilight and Celestia remained stuck in the land of the living. They spent countless years there, talking, thinking, sleeping. Twilight used dream spells she had learned to keep them mildly entertained, but one runs out of ideas eventually.
Celestia didn't go suddenly, but slowly faded as the sun became a nebula. She admitted being excited about the prospect to Twilight, excited about her escape. Twilight agreed with her, assuming that she would die with Celestia. Not knowing her true fate.
It had taken years to sink in. Years for the horror of her eternity to fully manifest itself in her mind.
Her patron was magic.
She was magic.
As long as something with magic existed, she would exist. As long as she existed, something with magic existed.
She was a self-perpetuating immortal. No matter what happened to her, no matter what horrors she underwent, no matter how long she waited, she would live.
She was adrift in empty, suffocating space, with nopony to talk to, nothing to think about. The wonders of the universe eventually blurred in her mind, as did her own concept of time.
She wanted to die. It was all she wanted. Everybody else had gone already, so why couldn't she? Why couldn't the world just let her stop being, let her stop existing, let her rest, let her stop, let her end-
But it wouldn't happen. It couldn't.
She wanted to go insane. Her body might not die, but Twilight Sparkle might be able to. She could be reduced to a lunatic with no concept of self. Years ago, the prospect would have seemed horrifying. Now, she begged for it.
That showed no signs of happening. Be it due to her princesshood or some cruel joke of the universe, she was as sharp as ever. As aware as ever of the crushing emptiness of her existence.
The universe slowly spread apart. Planets and stars and asteroids slowly drifting away from one another as the universe expanded. Eventually, there was billions of light years between any two atoms. Existence itself was dead.
Sitting in the middle of it was a purple unicorn. A unicorn who could never die.
A unicorn in hell.
Forever.