The backstory of these characters take place in alternate universe, you can get the basic idea of that universe here: https://mlpforums.com/blogs/entry/25250-solar-silver-oc/

Over thousand years ago, a group of skilled unicorns were called to the castle of the two sisters, there they would face a change to their lives that none of them had even imagined.

They were told that a great danger was threatening Equestria, and that they would be the ones to save it, it was not told what this threat was, as it hadn't yet been figured out in detail, but the princesses and the protectors of Equestria all agreed that the vision that had warned of it was genuine. How would these unicorns save Equestria? By becoming alicorns to be ready to take the rule in case something happened to the ruling sisters.

They were transformed and trained over the next few years, none of their's power matched that of the sisters, but they would have been able to take care of the necessary duties together if the worst came to pass. 

For a while the great danger didn't come, threats like the pony of shadows were dealt by the army of alicorns without too much trouble, and the Crystal Empire, while King Sombra's rule was unarguably evil, he didn't pose enough of a threat to rest of the Equestria for the princesses to get involved. But eventually, what was foreseen came to pass, but it was worse than anyone expected.

While the alicorns and the guardians of Equestria were busy elsewhere, Luna's envy over her sister reached a boiling point, and she transformed into a being calling herself Nightmare Moon. When the protectors of Equestria learned of this, they immediately headed back to the castle, but it was too little too late.

Celestia was no more, in her place there was being with flaming mane, she called herself Daybreaker. The two beings were fighting each other when the backup arrived, but they stopped and focused their attention to the new arrivals instead. It was hopeless, the two were too powerful, the original protectors of Equestria among most of the fresh alicorns were destroyed, leaving only few survivors, who only survived because they fled.

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Queen Noctuna

Personality and morals: Noctuna is generally a good pony, her only true wish in life is to settle down and live in peace, and as a ruler she's also shows that, wanting everyone else to live in peace too. Unfortunately fate hasn't been good to her, and she has grown bit twisted, she isn't evil per say, still wishing good for everyone, but she is volatile, and can go into extremes if she feels she has been wronged. There are also certain contradiction with her, as an example, she never wanted to be, and still doesn't want to be a ruler, but at the same time, she may lash out if someone challenges her title. 

Abilities: Being an alicorn, she's a powerful magic user, but for her specifically, she has the ability to communicate with and to certain extend control beings of the night. Beings of night meaning any animal, monster, magical being, etc. thing that is in some way directly linked to night and darkness, stronger the link, stronger her control.

Backstory in short:
Noctuna fled west, leaving Equestria for good, building a farm and settling down in what back then was middle of a forest somewhere outside Equestria. She was alone quite a while, enjoying her peaceful life, she had never wanted to take part in saving Equestria or any other kind of violence, fate just had had different plans for her. She promised to herself that even if the Equestria's problems would bleed outside Equestria, she wouldn't take any part in it, too bad promises are easily broken. 

After half an year or so, when night settled over Noctuna’s farm for good, it made a hard winter with her crops not surviving without sunlight, but due to combination of nature's resilience and magic in the cursed night, next spring new plants replaced those that didn't survive, and with some trial and error Noctuna found new crops to grow.

In the next few years refugees from Equestria started to arrive, settling nearby Noctuna and forming a small village around her farm, due to being an alicorn, she became the leader of what eventually grew into a town, she didn't necessarily wish for it to happen, but it was expected. 

The flow of refugees stopped eventually though, and considering what she heard from those who had fled, she believed the reason was something that the sisters did rather than a choice of those still living there.

After that, there was a long time of peace, the town grew into city and soon it was a small nation with smaller settlements rising near the original city. Along her nation, other nations were formed and grew around her, there was wars and other things, but Noctuna stayed out of them, her small nation was small enough for the others not care about it, and she had no interest in their matters.
 

But as time passed, Equestria became a threat to the outside word and the nations around her decided to join together to defend against the powerful sisters, calling themselves 'The United', Noctuna stayed out of it as she did not want to get involved with Equestria anymore, this unfortunately brought its own problems.

A dragon she had known when she was living in Equestria appeared on her door, warning about The United planning to take her nation by force, he explained they had already done so to another nation he used to live in, and offered to help defending against them. He said it was because her refusal to join the United made them believe that she was still loyal to her kind, or in other word, Equestria. She followed him to a hiding place where he and other loyalist of the dead nation were hiding after being declared criminals by The United.

She took a moment to think about the situation, she had promised herself to stay out of violence, but it didn't seem she had a choice with this one, she was the princess of the nation, it was her obligation to defend it. She didn't have an army though, she never build one for the nation, and while her old friend had some troops under his name, it wouldn't be enough against the forces of the United. She got herself armed and headed to the night, calling all beings of night to aid her.

The day for the battle arrived, and though it was chaotic, their side was victorious. Noctuna was exhausted and her so called allies used it to their advantage, as they celebrated and set up her old friend as the new leader, she had been betrayed. She lashed out, declaring her disagreement and sending her forces to fight the traitors while taking on her old friend personally. Being already tired from the previous battle, she had to flee eventually, she fled quite far and settled down again, again promising herself that this time she would stay in peace.

But again fate had other plans, as she got a message from one of her last friends, warning her that the United and her old nation were after her, they were going to bring him in front of a judge, but her they wanted dead. They send a small army to get her, threatening her friend’s life if she didn’t surrender, and while she was ready to fight, she couldn’t let her innocent friend to get hurt due to her. 

She was ordered to be executed by crushing, she was tied down with enchanted ropes and weight raised above her, but to her fortune her enemies didn’t understand Alicorns. They broke her horn and destroyed her cutiemarks, but that didn’t stop her from using magic, and when the panic first set in, she blasted her way free an ran, though not before taking down some of creatures around her. As she fled, she decided that Princess Noctuna was now dead, she instead adopted the title Queen, purposefully leaning on the evil past of the title, if the United wanted her to be the villain, she would be one. 

This little event left quite a mark even after healing fully, her horn was broken and her flank and legs were badly scarred, though luckily the first one was possible to fix, as the technological advancement had made huge leaps during Noctuna's lifetime, and even as an outlaw, it was relatively easy to find a replacement horn. 

During her time healing, Noctuna found more potential allies, creatures wronged by the United, and she started planning her revenge. She sneaked back to her original home, and while it had been looted and destroyed, few hiding places hadn't been found, and she gathered equipment she had left there. When she returned, her new allies had decided to abandon the plan, but she didn't let it stop her and she proceeded with her plans. 

She made herself a crown, collar and shoes fitting for a Queen, and headed to the darkest shadows of the night to gather the darkest beings and most terrible nightmares she could find, after which she marched to her old home, and sent the horrors of the cursed night against her old subjects, she was there to destroy, she build this nation, and she would also destroy it. It wasn’t until nobody was left to fight back she paused, she could already hear the backup armies coming for her, enough troops to take down a whole nation, but she had one more trick in her arsenal, she took out an ancient scroll, once given to her by Star Swirl the Bearded himself, to use in case a villain would cause too much trouble, well, now was the time.

The door of the ruined building she was in was kicked in, she turned to see the soldiers and smiled, she then lit her horn and vanished, she banished herself, where she may end up she didn’t know, but anything was better than what she left behind.

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The Crystal Queen Evenide

Personality and morals: Evenide is what one could call cold, the few emotions she feels are weak, but this doesn't make her evil, as she still has the sense of good and evil that has been taught to her controlling her actions, it just isn't linked to her emotional state and rather is purely question of principle. She doesn't always succeed in being perfectly good, and there isn't conscience bothering her if she accidentally does something evil, but she tries her best. 

Abilities: Besides being powerful magic user, she has the ability to "spread" crystals on pretty much anything, and she has a special connection to those crystals in form of a sort of a hive mind. On most things this only means that she can hear and feel things from near these crystals, but if she "spreads" them on a living creature, she gains control over that creature. The downside is that if these crystals are damaged, she can feels it as pain.

Backstory in short:
Evenide fled north with Diurna, but they separated when they reached the Crystal Empire, because while Evenide decided to stay in Sombra’s realm, Diurna couldn’t make herself to look past Sombra’s evil actions, and decided to continue on her way.

Evenide, while she tolerated Sombra enough for enemy of your enemy is your friend to stand true in current situation, she couldn’t watch the suffering of Sombra's subjects, and instead of settling in the city, she settled in the frozen wilderness of Equestrian north, claiming a small cave as her home. She didn’t have much time to live there though, as in half an year or so, things changed, one morning, or at least what she assumed was morning, the world had been split in two, on one side there was day, the other, there was night, the unnatural way it was spit told of magic’s involvement, and it wasn't hard to guess who was behind it, it seemed the sisters were having a power struggle.

This however wasn’t what changed things for her, in fact, it was quite inconsequential to her all things considered, she was glad about the potential of weather warming up in the future though. It was what happened few hours later that had more drastic effects, from the capital of the Crystal Empire a blast of dark magic spread out, covering the entire north, and few minutes later, crystals of all sizes and colors started to grow out of the ground, first surrounding the city, then spreading out.

Some crystals reached the height of mountains, and though Evenide tried to flee, their spread got faster and faster, and soon it reached her position. The crystals grew around her, and Evenide thought she was safe, but then she felt something inside her, it started as small aching, but soon it turned into sharp pain, and in matter of seconds she was laying on the ground in agony. She didn't know how long it lasted, but it felt like eternity, and before she could figure out what was happening, she passed out from pain. 

She woke up in a bed, crystal ponies moving around her, it took her a moment to realize where she was, but she guessed it was a hospital. The ponies around her seemed surprised when she sat up, surprised and terrified, they looked at her like they had seen a ghost. She looked at herself, and the reason for their fear was not hard to tell, she was covered in crystals, no, not only covered in, she was crystals, they were coming from inside her, she wasn’t a doctor, but even she could tell that by all logic and reasoning, she should be dead, or at least in great pain.

She wasn’t shocked though, in fact, she didn’t really feel anything regarding it, it was unexpected certainly, but she couldn’t find it from herself to freak out, it surprised her, as in her mind she knew she shouldn't be so calm. She realized it wasn't only thing wrong as she looked around at the ponies near her, they were in poor shape, as to be expected from living under Sombra, there was also pair of his mind-controlled guards at the door, it was something that not long ago horrified her, but now, while Evenide could acknowledge it to be horrible, she couldn't make herself to feel anything.

After some time, the King himself appeared. She doesn’t remember exactly what words were used, but Sombra offered her a place on his side, as his queen. While her emotions had been greatly reduced due to the transformation, her sense of right and wrong hadn’t gone anywhere, and she disliked the idea, but regardless of that, she accepted the offer, as she didn’t see any other choice at the moment. She ruled with Sombra for some time, not that she really had any say in anything, and eventually she got enough of watching from the side as the crystal ponies suffered, and when Sombra least expected it, she turned against him and took him down, taking the rule of the Crystal Empire for herself.

The citizens were wary at first, but they soon learned to like their new leader, Evenide made lot of things better, abolishing the mind-controlled guard and giving the ponies their freedom back.

Unfortunately the isolation and Sombra’s rule had had its effects and everything couldn’t be fixed, the population was on steady decline, and there was no way to stop it, and after some generations, she was alone, a ruler of empty empire.

Being alone and with nothing to do, Evenide moved into the library in the Crystal castle, studying Sombra’s personal study, and eventually she found a possible solution to her growing loneliness, a spell Sombra had planned to use if the sisters ever reached the Crystal Empire, a spell to make the whole city disappear. Evenide studied the spell, making some changes, and eventually used it on herself, sending herself to a dark timeless void, until somewhere, and sometime, something pulled her out.

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The Fire-Monarch Diurna

Personality and morals: Diurna is the most morally pure out of the three, even if the most evil looking, her transformation has had its effect on her mind too, making her bit more hot-headed, but as she acknowledges this change, she actively tries to combat it, and any harm she does to others is mainly accidental.

Abilities: Besides magic, she can create and control fire, and due to her being literally hot, she can also create (and control) magma, molten metal, and any such extremely hot material by melting it. 

Backstory in short:
Diurna fled north with Evenide, but they separated when they reached the Crystal empire, because while Evenide decided to stay in Sombra’s realm, Diurna couldn’t make herself to look past Sombra’s evil actions, and decided to continue on her way.


She took flight as she reached the end of Equestrian land, flying over seemingly endless sea, and just as she was staring to run out of strength, she saw land. It seemed to be a mountainous, highly volcanic land-mass, not the most pleasing place to live, but land was land. When she landed, she collapsed right away, she was exhausted, and she had injured her snout quite badly in the fight against the sisters, so she had barely any strength left to even stay awake, thus she laid there for a while.

She made a makeshift fix for her snout after she had rested for a while, she used pieces of wood she found at first, but she eventually found metal and made more permanent and refined fix. 

With further exploration, she found out that the land was inhabited by hostile dragons with different build to those in Equestria, but due to the various caves all around the landmass, she managed to find a place to settle where the dragons didn’t find her. She kept exploring the land, and some time later she found a different species of dragons from the eastern part of the continent, they seemed to be less hostile, but there was some kind of magical force stopping her from entering their land, and the dragons ignored her regardless of her attempts to get their attention, so she had to return to the cave she had taken as her home.

Luckily to her, even in this fiery and hostile environment, there was plenty of plants to eat, though it would lead to something she didn’t expect, as after few months of eating the plants of this place, she started noticing changes to her body.

It started with her coat turning hard and rough, and after accidental encounter with molten lava, she learned the new coat was fireproof, her skin burned still though, so she got bit of an injury from that.  After more months it got more extreme, her skin got covered by fireproof scales like layer, which solved the burning problem for good, and her mane and tail literally got hotter until they resembled fire and magma. The changes continued getting more extreme until eventually she had transformed into a different being, she was still in a shape of an alicorn, but parts of her body were permanently on fire and her saliva was like molten metal.

She spend some time in her new, fiery form, but eventually things started to happen, several new, seemingly highly magical dragons suddenly appeared, they were huge, bigger than Diurna had ever seen any living creature be, but they were not interested in her, no, she was like air to them, instead they were gathering the dragons native to this land somewhere, where, she didn’t know, as any time she tried to follow them, dragons, seemingly made out of pure magma stopped her, they didn’t try to hurt her, they just didn’t let her through. Her magic also seemed to be blocked by something far more powerful than her, so there wasn't much she could do to the dragons blocking her.

She left the matter alone, kinda happy that she didn’t need to worry about the dragons anymore, but it didn’t last long, as one what she assumed to be night, she woke up to a loud sound, like the continent itself had started moving, and it had, the mountains surrounding it had started to crumble, water from the sea started to flow in, but before she could react, everything went silent, and dark.

She floated in nothingness, she saw nothing, she heard nothing, she felt nothing, it was cold, yet it wasn’t, she was there for eternity and only for an instant, her head was empty and yet so full of thoughts, was she dead? Or did she exist at all? Then suddenly there was light again...