I remember an old story where unicorn Twilight was in a life or death situation in a cavern or something, possibly injured, or just very angry. It described how most unicorns have a small pool of Mana, but in her desperation Twilight digs deeper and finds an endless ocean of it. Possibly related to being the Element of Magic. She taps into that power and ascends to an Alicorn (though the story may have been written before that term was popular, so she just grew wings/power). Then she has some flying combat in the cavern and dominates completely.

I also think it was a TwiLuna story. Decently long, too.

The main emphasis I remember was the tapping into an endless ocean of mana.

there were alot of stories like that at one point, don't remember reading one exactly like that, though.

Maybe Sharing the Night?

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I don't think it was this. I read it again midway through 2021 and it didn't have the scene I remember. Great story, though.

7624520
Well, it sounds like a good story! Let me know if you find it.