The Alicorns of Old

Art by Slouping for my story Fallout Equestria: Fallen Angels ; thank you for everything, my friend.

Essence
Basically, they're all immortal beings of great magic and they've been around since the world started turning. They are beings just as ponies, griffins, dragons, parasprites etc. Their numbers were quite small (just about 200) with eight stronger ones and two strongest ones, but there were several.

In an analogy to the solar system, the two hundred were named Phengary (Moons) and the eight more powerful ones were named Uranoi(Skies; planets). The two were named the two Cosmoi (order; rules), who were able to do some rudimentary thinking.

They are immortal due to the vast amount of magical energy in their spectral body. But they are not immortal immortal. If the magic is somehow completely drained from their bodies, the turn into inorganic matter, which could be described as alicorn stone statues. However, they are not dead. Alicorns drain their power from the world itself, taking it in by all kinds of energy. But the most important and powerful one was the light of the sun they absorb at day and during the night via its reflection of the moon. Meaning even if one were able to somehow empty out their body of magic, they would restore themselves slowly but steadily until they were "alive" again.

In the beginning all of them were just there. No purpose was given to them, they basically where just like Discord, just without motivation do evil or good things. They had no motivation at all, leading to them just vegging out (Not fully fitting, because they don't die or go away) will life around them went on.

Magic:
They had simple but powerful magic, connected to the Elements of Harmony/Six (main) Virtues, but without motivation to do anything, it was basically wasted and rarely used if someone not-pony attacked an alicorn for some reason. (Ponies were innocent and harmless beings back then, so there was no need to turn alicorn magic against them. Manticors, dragons and drgaonequi on the other hand... if they decided to attack, that alicorn would respond).

Each virtue/element gives them access to a certain spell/path of magic:
the Element of Kindness allowed them to create powerful shields around themselves while the Element of Honesty for example allowed them to control the earth they stand on to smash an enemy (brutal Honesty can hurt like a punch in the face) and the Element of Loyalty to bind other beings with magical chains. Loyalty is kind binding after all

Each of those primordial alicorns has the magic of a certain amount of "elements" at its disposal (usually two) but due to their lack of involvement and guidance, they the virtues developed by those paths of magic didn't really kick in ever. They remained vegging out until...

The Twist:
...one day, two of them discovered a great secret by accident: the Elements of Harmony in their graspable form, the crystals/necklaces we know them as today. The sudden power and all the virtues crashing onto them at once made them as psychologically as close to a pony as possible: they became emotionally involved and aware of their surrounding's interpersonal effects. Those two alicorns became Celestia and Luna.

Beings with extraordinary power even compared to their alicorn brethren and emotional involvement with their younger siblings: the ponies. And they didn't like how the ponies were treated in this world: dragons, manticores and hydras fed on them, griffins and buffaloes killed them if they stumbled into their territory and beings like Discord or Tirek played cruel pranks on them and stole their magic.

Alicorns are strong - considered the most magical beings in the entire world by some - strong enough to take on manticores and hyrdas with ease, even able to defeat a dragon, but even as highly powered as Celestia and Luna were with their new found Elements of Harmony, they couldn't be everywhere at once to protect the ponies. So they needed reinforcements or some kind of police, powerful enough to establish "peace". And they had reinfocements in form of two hundred more alicorns, each powerful enough to protect the ponies in one spot.

But they saw no reason to do anything. What point was there in opposing nature?

So Celestia and Luna had to establish their power somehow. Just beating the others to a pulp wouldn't suffice, since they were immortal. So they had to get a "leverage" to get them to "cooperate". As mentioned above, the alicorns as magical beings draw their power from the world around them, especially by the sun's light (and the moon's reflection of it by night). So they did what they could to get to the pressure points of the alicorns: taking the sun.

With the powerful magic they had gotten by acquiring the Elements of Harmony, they forged a "contract" with the sun and the moon (figuratively spoken of course; the sun and moon can't bargain or anything; they simply bound their own being to those celestial bodies) and from no on were able rise the sun and the moon, and therefor they were able to "blackmail" the other alicorns: do as they said or they'd loose their powers. And there was no a damn thing all the alicorns could do about it. All but two of them.

The two most powerful beings, named after the driving forces of the universe, Dyname (Power) and Arete (Virtue), still found it weird go against nature like that. And they had the power to oppose Celestia and Luna. And so the First Alicorn War began. A war that lasted a few days until one of the Kosmoi, Dyname, decided that Celesita's and Luna's was worth a try. Arete, the one driven by his virtue and even stronger loyalty, wanted to stay true to the creation, hence keeping to oppose the two new alicorn rulers, resulting in a stalemate. But then Dyname made up his mind and allied with Celestia and Luna, who were now able to overpower the last standing alicorn. Having the control about their powers and the strongest of them all under their thumb, Celestia and Luna were able to persuade the other alicorns to do as they said.

And thus began a first era of peace and order in the pony lands, latern known Equestria.

The First Era of Peace:
Now able to help all their pony brothers and sisters, Celestia and Luna began their campain against everyone and everything doing harm to the ponies. With their and the other alicorns' powers - especially the ones of the two Kosmoi - they brought the dragons and dragonequi to the edge of extinction, smote countless pony-vorous monsters and drove back the buffaloes and griffins into their own territory. But their minds thought like a pony's, like a that of a peaceful creature, so they didn't just kill them all to solve the problem forever.
So Celestia and Luna took a territory of their own and formed a country. Not yet the Equestria as we know it, but a country were ponies could live their lives and were protected by the near-almighty police-force of the alicorns.

They started to rise sun and moon to give the ponies a schedule to play and to sleep, but also ensureing their army didn't ever loose its power to protect. And said army was happy to obey. They had a purpose now and they didn't question it. Questioning wasn't their nature. And even Arete grew to accept it. After some time, the modern Equestria was formed by the unification of the three tribes and everypony was happy.
Almost everypony...

Nightmare Moon
Luna was thinking just like a pony.
And therefore she could grow jealous too.
Jealous of the ponies who had fun by day, jealous of the day itself, since everypony apparently liked it better than her night.

So - in her hurt Loyalty to the night - Luna decided to change Equestira's laws so that ponies would start to love the night. Celestia tried to negotiate and in the end even considered to take down Luna by using their army's power to imprison her forever, but it was too late. The alicorns - internally changed a tiny bit by Nightmare Moon's Loyalty's outrage - had been already influenced by it. A tiny yet poweful spark was lit in them, the spark of Loyalty. And this Loyalty drove them to choose sides.
One group sided with Nightmare Moon and Dynamo, who understood why Luna was angry, in an attempt to establish an equilibrium of love for night and day, and be it by force, while the other ones rallied with Celestia and Arete, who were eager to protect the order they had established. Thus the war began and with it, a dilemma occurred:

Neither side was capable of taking out the other's powers without becoming powerless themselves and non of the alicorns was capable of dying either. But they tried. The resulting battles lasted a long time and scarred the face of Equestria, leading to nothing but pain and suffering for the ponies in Equestria. And no end could be seen, not ever. The fighting would continue forever as would pain, suffering and death for the innocent ponies.

The alicorns didn't see any problem: there were evil alicorns who opposed their train of thought and therefore, they had to be eliminated. Everything else was secondary. But Celestia and even Nightmare Moon still had pony minds and saw what their dispute caused day in day out. And while neither was wanting to let the other one win, they agreed that an end had to be put to the earth-shattering battles. They wanted to find a way of taking out the alicorns, so they could settle this one-on-one.

But how to do that? Alicorns were immortal and even if they managed to take out a large number, others could simply "revive" them by giving them a bit of their magical energy, only to regenerate that bit almost immediately. They would have to take down all alicorns at once and then cut them off from any power they could use to regenerate. No task that could be accomplished by the means of this world. But then again, some say Nightmare Moon had have contact to something beyond the skies...

From an unknown place, the two alicorn princesses had managed to get a piece of metal that had an unique and deadly ability, something that didn't really belong into Equestria. The material called Moonstone - in opposite to the energy producing sun - was able to absorb relatively small amounts of magical energy and simply make them vanish. The perfect weapon for the princesses' needs.

With a plan in mind, they declared that a final battle should decide the war's outcome, well knowing that this battle would never end. And at the battle's peak, when even the two Kosmoi were a bit weakened from battling each other, the two princesses knifed their own troops in the back, using the Elements of Harmony to temporarily defeat each and any other alicorn, turning them into stone statues. With their strength combined one last time, they took the statues to special places where the stream of natural energy was weakest to slow down the alicorns' regeneration. And to delay the process even more - almost stopping it - they rammed the pieces of Moonstone into each petrified alicorn.

In the last hours of the two princesses' alliance, they decided that whoever would win this war would look after the petrified alicorns, making sure they and their war never returned to Equestria.

And what happened afterwards is well know...