(Note: this story is a one shot. It based on the TV series, the chapter books and the comics of My Little Pony, so the following events could contain some as head canon/non canon. And also note that some events could contain theories.)
The Crystalline Tail of Crystal Empire
Many moons ago, in the far northern depths of Equestria, there was a barren land that was covered in snow. It was caused by the spiritual beings called the windigos, due the negativity among the ponies who bickered and fought over fruitless situations. The food was scarce and the warmth within their homes was barely limited. The ponies felt hostile to one and other, but not all of them showed hatred.
One particular pony, a unicorn named Amore, had shown kind and compassion toward them, despite of their stubborn scoffs toward her. Somehow, she was very special as she was stated to be a unicorn full of love from a young age.
The winters were getting worse and most of the ponies began to freeze along with their cold hearts, thanks to the windigos. But one night, as the blizzard blows harder, with a few remaining ponies by her side, Amore traveled north toward the Crystalline Mountains in hope to find another land that had yet to be touched. On their way, they were saddened to see that ponies and animals were frozen due to their cold hearts. Then, as they climbed on the mountains, they discovered a cave where they noticed beautiful crystals glowing as they formed. They continued walking down the cavern until they see a beautiful heart-shaped crystal. Somehow, Amore sensed a powerful love from it and approached it. And to her shock and amazement, she suddenly felt an immediate connection to it.
The power of the crystal began to shine as if it listened to her warm heart. Then, the crystal had spread its magic across the land to wipe the snowy darkness away along with the windigos. The ponies and creatures became unfrozen and were amazed of their crystalline forms and the mysterious light.
Emerging from the light was Amore and the ponies that followed her along with the heart-shaped crystal. She even had her own cutie mark due to her love and connection to the crystal. The land was turned green and beautiful as the Crystal Empire was formed and protected by the powerful love of the Crystal Heart with Amore now as its princess.
Many moons have passed. The Crystal Ponies were living their lives full of love, happiness and compassion as the empire was ruled by Amore. And they could've been more grateful as they grown their food and give each other warmth and comfort.
One day, Amore had planned on creating her own festival to give more love and happiness to her subjects in order to empower the Crystal Heart. However, it was stolen by a greedy dragon and it was taken back to the Crystalline Mountains. Without the heart, the empire was left vulnerable and the Crystal Ponies were in despair. Amore tried to use her powers to spread love for them, but it wasn't enough.
Then, Princess Celestia and Princess Luna, along with the griffons traveled to the Crystalline Mountains to confront the dragon and retrieve the heart. With the heart restored, the Crystal Ponies' spirits were lifted as they celebrated their very first Crystal Faire.
More moons have passed, and there hasn't been any threat to the Crystal Empire. Until up to that dark day…
An orphaned unicorn colt named Sombra was found in the frozen wasteland north of the Crystal Empire and was brought to live in an orphanage. He was considered bright, yet not sociable with other foals since they teased him about his matter of speech and the lack of a cutie mark. The first and only friend he ever made was a unicorn mare named Radiant Hope who was also teased by the other foals.
As they saved up their gems for the upcoming Crystal Faire, they gazed upon the Crystal Heart at the palace. The heart revealed the visions of their future. Radiant Hope sees herself as a princess, and Sombra sees himself as an evil being of darkness. Princess Amore sensed Sombra's vision, telling him that he has the power to change the future.
The Crystal Faire arrives, but Sombra suddenly had fallen ill, which only worsen as the Faire goes on. Radiant Hope decided to stay with him, missing out the Faire. The next day, he was healthy again. However, each year, Sombra suffers the same pains and aches over and over, whenever the day of the Faire arrives. It came up to the point when his condition worsens so much, his body began to crystallize and fracture. Then, Radiant Hope who was desperate for him used her magic to heal him. Afterwards, she gained her cutie mark as she discovered a special talent of healing.
Radiant Hope's talent gained Princess Celestia and Princess Luna's attentions, and they offered her to study with them in Canterlot. As her path to becoming a princess was coming true, Sombra realized that his dark destiny is also coming true, so he ran away into the frozen wasteland outside the empire.
On his way, he discovered a strange red crystal which speaks to him, calling itself mother. It revealed to him that he is not a pony, but an umbrum- a shadow and smoke given physical form. He was told that he was created to destroy the Crystal Heart, because it is a weapon against the darkness and could set free an army that sleeps below the city. The red crystal then awakens Sombra's dark powers, transforming him into an umbrum.
Sombra returned to the castle to steal the Crystal Heart, but Princess Amore arrives to convince him to change. But Sombra enraged for a fact that she knew his destiny all along, turned her to stone, shattering her in pieces. He then hid the Crystal Heart somewhere so it couldn't be used against him and declared himself as the king of the Crystal Empire, enslaving all of the Crystal ponies.
Radiant Hope, shock and saddened of his sudden betrayal, goes to Canterlot and warned Celestia and Luna of his rule.
Sombra continued his cruel reign as the Crystal Ponies fell into despair and fear. Until Celestia and Luna arrived to stop him. After his defeat, he realized that Radiant Hope had told them of their tale. Before his demise, he decided to place a curse upon the Crystal Empire. Radiant Hope and the remaining freed ponies, along with Amore's relatives were forced to watch the Crystal Empire vanished as the snow continued to bury the deserted land.
The kingdom of the Crystal Empire's disappearance was spread around within history as the memories of the empire were long forgotten before King Sombra's reign. It was facing the dark ages within the cold darkness until one day, it was discovered once again.
More than a thousand of moons have passed, ever since Nightmare Moon's banishment to the moon. There was a young infant Pegasus named Cadence who was found abandoned within the woods near a village. She was adopted by two earth ponies to be raised as their own. As she grows up within the village, she was told that she was very special and she had so much love to spread and share with other ponies. She received her cutie mark after discovering of spreading love and compassion. She had been continuing to do to so, though it didn't last long.
One day, an evil pony enchantress named Prismia had begun draining the love sources from other ponies with her powerful necklace. She had been jealous of Cadence who was adored by them. She wanted to have all of that power and attention to herself.
Then, Cadence bravely went up to Prismia and used her healing love to comfort her and lifted the curse that the necklace had caused. The necklace was then destroyed due to Cadence's compassion, and Prismia learned the error of her ways as she now knew the true meaning of love.
Suddenly, Cadence was transported into a strange place where she had met Princess Celestia for a first time. Celestia had explained to Cadence that she had watched over her ever since she had discovered her talents of sharing love to other ponies. She knew that Cadence would be one day become one of the true leaders to rule her own land.
Cadence was then transformed into an alicorn as she was given a horn along with the magic abilities. And she was even declared as a new princess with a new name; Princess Mi Amore Cadenza. Celestia then took Cadence in as her niece and teaches her at the Cantorlot Academy.
There, as she studies, she met and fell in love with Shining Armor, a brother of Twilight Sparkle who she was foal-sitting.
Shining Armor asked Cadence to marry him and she said yes. Before the events of their Cantorlot wedding, she was kidnapped by a changeling queen called Chrysalis who imposes as her to seduce Shining Armor and starts draining his unicorn energy. This charade was eventually discovered by Twilight Sparkle as she was sent down below by the evil queen, only to find the real Cadence there.
They both stopped the wedding in time and Chrysalis revealed herself, starting an invasion with her army of Changelings.
When all seems lost, both Shining Armor and Cadence used all of their powers of love to send the queen and her changelings away. Not only the wedding was restored, but it was renewed. And they're both happily married.
After their marriage, the Crystal Empire was finally discovered. But all of the Crystal Ponies who resided there were in despair and fear as the upcoming Crystal Faire was approaching. Also, not only the Crystal Empire returned, but so did King Sombra in his terrifying umbrum form.
The Crystal Ponies were desperate for the light of the Crystal Heart as they greatly feared of the evil king's return. Twilight Sparkle and her dragon friend, Spike discovered the Crystal Heart and Cadence reached it before the king. She placed it back in its rightful place, encouraging the ponies not to give in to fear and despair as they worshipped their new empress of the Crystal Empire.
The empire was revived in its much powerful light as the Crystal Heart had given the citizens hope and happiness and King Sombra was finally defeated.
The celebration occurred as the Crystal Faire arrived with Princess Cadence and Prince Shining Armor as their new rulers of the Crystal Empire.
The Crystal Heart also plays an important part of the Crystalling ceremony for the newborn foals in the empire. Cadence had given birth to an Alicorn filly named Princess Flurry Heart whose magic was very powerful.
The little princess's scream had caused the Crystal Heart to scatter in pieces and the blizzard storm clouds appeared because without the Crystal Heart, the Empire would become a frozen wasteland due to the curse of the windigos and the Crystal ponies would once again become depressed, fearful and hopeless.
With a help of a young wizard pony named Sunburst as Flurry Heart's Crystaller, the Crystalling was a success. The light and love within the witnessing Crystal Ponies is connecting into a single crystal which Sunburst then merges with the Crystal Heart. It was fully restored as one and it dispels the storm.
Up to this day, Princess Cadence and Prince Shining Armor had continued their rule as they cared for their daughter who would one day rule the empire herself. The love and light from the Crystal Heart continued to be spread and shared among the Crystal Ponies even toward the outskirts of Equestria.