Alrighty my take on the process of Alicorn ascension. First thing, for a pony to become an Alicorn they must follow three factors, failure to have hall all three means they will not ascend.

1) Bloodline:

They must be a decedent of the First Alicorn who lived during the HWE-era, this Alicorn is in turn a decedent of Laurel the Blessed (who is in some metaphysical way the Goddess Epona herself).

2) Divine Blessing:

The deity that rules over their domain will bless them with their powers and warn them that their ascension is near. This is done in the form of a dream.

3) Emotional Stress:

Heighten emotions causes the ascension process to kick into overdrive. It doesn't matter if it is a positive or negative emotion.

Celestia and Luna are descended the eldest foal of the First Alicorn, and both were born Earth Ponies. Celestia received her blessing from Helios, God of the Sun, and extreme rage at Discord caused the emotional stress. Luna got her blessing from Selene, Goddess of the Sun, and fear was her emotion. Cadence was actually the youngest foal of the Alicorn and was born a Pegasus, she recived her blessing form Caelus, God of the Sky, and her emotional stressor was Love. Twilight is decendent of the middle foal, an Unicorn, her blessing came form Janus, God of the Veil and Magic, her emotional stressor was Joy.
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MALFROST: You sound like you have a pretty well developed headcanon. I like your idea of the three requirements for becoming an alicorn, but that seems to strengthen the argument that only certain ponies will ever be able to achieve alicorn status and that they are infact like decedents of gods themselves, giving strength to the idea of Uberpony, a very Nietzsche like view on the subject. I'd like to know more about your backhistory and religion that you developed if that is alright!
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Yes only certain ponies can become Alicorns, but they are not Nietzchen Uberponies. If anything they would buck those ideas where it would hurt the most. They are also not all-knowing or all-powerful, they may have power but to use it would mean a lot more destruction than what the bad guy is causing. Plus they are still mortals... Um... I have put together a chart on Alicorn power tiers. There are four tiers, each one has been seen in canon.

Tier 3: Lowest level power, this is the Alicorn's birth form. For the most part it is not different from a normal pony except they may have something special in regards to their elemental alignment (which of the Four Element + Magic are the aligned to).

Tier 2: Base Alicorn, pretty much they now have horns and wings but that's about it. Cadence, S1 Luna, and Twilight are Tier 2 Alicorns.

Tier 1: Full Realized Alicorn. They are now fully attuned with their domain and could be considered demigods. They are characterized by their flowing manes and tails. Celestia and S2 Luna are all Tier 1.

Tier 0: Fully Synced. They have now gone beyond their mortal form and now living gods and the Avatar of their patron deity. Extremely powerful. Characterized by their manes and tails acting like smoke or fire, iris color bleeding into the whites, dagger shaped pupils, glowing coats, and speaking in the Royal We. Nightmare Moon is a Tier 0, though a false Tier 0.

And Alicorn can move up in the Tiers but can also move down. In the case of extreme injuries they can even descend to their birth form, this was the case with Celestia during the War of Eternal Night. Their magic will heal them but they cannot use it.

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Ok I'm hearing all this talk about Celestia manipulating events so that Twilight can ascend. Guys look at it this way, she is a teacher, and a teacher always wants their student to better themselves, to be all they can be. Also while Celestia may have given her the book, Twilight still went forward and proved herself. It wasn't the spell or her teacher that caused her to ascend, it was all Twilight feeling the joy that she had shown they she can do something that not even Star Swirl could do. Think of "Celestia's Ballad"

" You've come such a long, long way
And I've watched you from that very first day
To see how you might grow
To see what you might do
To see what you've been through
And all the ways you've made me proud of you

It's time now for a new change to come
You've grown up and your new life has begun
To go where you will go
To see what you will see
To find what you will be
For it's time for you to fulfill your destiny"

It was all Twilight
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MALFROST: So in your head canon, Alicorns acquire immortality when they reach tier 1 or 0? Or are you just saying that Alicorns are never immortal, they just have longevity? Much like elves in LOTR, they can never really die of old age, but they can be killed in combat and die through injury ect ect...Because that would be a really interesting distention to draw, especially if war were to ever descend back onto Equestria.
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Perhaps when they reach Tier 0, but since that has never happened beyond a nasty case of possession it is hard to say. Comparing them to the Elves isn't too much a stretch.

I think the best way to put it can be taken by a line from the second chapter of "The Broken Mare": "My claim on immortality is a tenuous one at best. It is simply because I have not died yet.".

Equesrtia has been at relative peace for 200 years. and it takes a lot of punishment to even harm an Alicorn let alone damage one enough to cause them to descend. In the case of the War of Eternal Night, Celestia descended from 1 to 3 because she exhausted herself fight NMM, it was the Winter Solstice so she was at her weakest, and she fell head first into a iced over river before heading over a set of falls.

MALFROST: That's certainly an interesting way to look at it. It's like saying,"Well, I figure out if I'm immortal or not if/when I die." That's a very pragmatic way of looking at it, and that certainly is a very I could picture Celestia holding. So the way you describe it, really the only way an Alicorn to die would be to damage it while it is in its level 3 stage enough to where a normal pony would die. Of course it would take alot to get a level one or two down to that level I'm guessing.
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Well she does other things to worry about, and at being over 1500 years old she is a bit old for an existential crisis.

Yes, but even then it may not be so easy. There is more to the quote that I gave you, the rest is this: "This body still has the instinct of self preservation. I must assume there is a reason for that. I must answer any attack with the full force I am capable of, in defense of my life". This is completely true for Celestia and Luna is they grew up during the Discordant Era. Cadence, in my headcanon, is also a member of the Guard and can defend herself. It would take a lot to knock down a 1, and when that happens you know the shit is about to hit the fan. And when they ascend to Tier 0? Epona help you.

Each Alicorn has an Elemental Alignment that ties into what their Domain of power is.

Celesita: Fire and the Sun
Luna: Water and the Moon
Cadence: Air and the Sky
Sejr: Earth and the Earth
Twilight: Magic and the Veil.

Strange, I always thought Celestia was somehow linked to fire too.

...

That said, you let your brain work too much on his own

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It's not too far of a stretch given her connection to the Sun. Cadence is the hardest to comprehend her link to the Sky and Air

I wonder who is Sejr? And I'm guessing that Tier 0 alicorns are more wrathful than the lower tiers, which in turn make them act tyrranical by nature. I wouldn't want to meet a Tier 0 Celestia if my mere presence in front of her would anger her.

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Oops... knew I forgot something. Sejr is or at least was the the Alicorn of Earth, she is normally called the First Alicorn and lived during the HWE era of history (about 500 years before the birth of Celestia and Luna). She is believed to have been the only Alicorn to have died, and perhaps the only one to have ascended to Tier 0 status, both of which happened in the final fight with Tirek the Abomination (who tired to first enslave then wipe out the ponies).

Tier 0 doesn't mean they are tyrannical, Nightmare Moon was a fluke, they are still beings that one does not want to piss off. They are less mortal and more godlike. And to see one may mean that a great Trial is at hand.

Each Tier 0 Alicorn seemingly abandons their mortal name, to symbolizes that they are now the avatar of the gods.

Celestia: Sol Invita/Invitus (Unconqured Sun)
Luna: Artemis Noctiluna (Moon Night-brighter)
Twilight Sparkle:
Cadence:
Sejr: Terra Victoria (Victorious Earth)

To gauge how powerful they could be, Ghastly Gorge is not a natural feature, it was made by Sejr during her fight with Tirek.

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And to see one may mean that a great Trial is at hand.

Does that mean that your life would change if they meet you personally? Like, for example, damning you eternally if they see you as a life-ending threat, or turning you into a guardian if they see you worthy enough?

And, my God, if their powers is equivalent to an earthquake of high magnitude or a dormant super volcano or the Sun itself, then I would rather hug a manticore than meet an angry god-tier alicorn.

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More like that Forces of Evil are trying to destroy the world and enslave or kill everything.

You will have nothing to fear from them as long as you do not harm anyone that they care about. To understand how an Alicorn may think can be quoted from the book Wee Free Men by Terry Pratchett.

All witches are selfish... Then turn that selfishness into a weapon! Make all things yours! Make other lives and dreams and hopes yours! Protect them! Save them! Bring them into the sheepfold! Walk the gale for them! Keep away the wolf! My dreams! My brother! My family! My land! My world! How dare you try to take these things, because they are mine!

I have a duty!

Ah, Tier 0...

The perks: True Immortality and immense, truly god-like power over their realms.

The downside: Your morality scale goes off the axes of Lawful/Good/Evil/Chaotic, and goes straight for the Blue and Orange, under the best circumstances. You lose care for others and become completely apethetic, defeating the intent of why one might have gone "Tier 0", though not necessarily the purpose. You want to destroy, say, something like Tirek? You automatically go for the most grim and deadly solution.

Nuke him, to abridge it.

Does that all sound about right, Pal?

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Kind of, I wouldn't say the Zeros' loose all sense of morality or care, but they do become something more than mortal. Though I may use an idea from the STALKER/MLP crossover that the Zero is a feral state, or at least a berserker state.

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Makes sense, I suppose.

... I still recall that poorly-written PM of Tier Zero Luna I showed you. Speaking of... I hardly see you there anymore.

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Student

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Ah. Okay then.