Notice that Tirek was looking for three alicorn Princesses? That could possibly mean he was banished before Sombra ruled over the Empire. Which in turn means that there was an alicorn, the first Crystal Princess, at the Empire's head before Sombra came to power.

I figured this was the case for a while now, but it's always near to get a little confirmation, even if it is very subtle.

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Ok it seems I really need to to watch the episode

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My headcanon is that Cadence is just the most recent living incarnation of the Princess of Love, and that she resurrects to experience all of the types of love over and over, always ending up with Shining's soul as her partner. So, yes, I agree with your theory somewhat–I'd like to think there was an alicorn before her, too... but that it was her, in a way.

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I actually have a series that explores this. Or will, eventually. I too believe that there has always been a crystal princess in spirit, one way or another.
Um, if you are interested, that is.

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I have a one-shot in the works along that theme, but I'm always interested in hearing other's takes on ideas as well.

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This was my attempt at writing about how the stories of The Crystal Empire/Crystal Princess/Sombra/Cadance all came together: Through Ice and Shadow.

No downvotes! Well done, Pal! I'll have to check it out when I get time!

I always figured there was a Crystal Princess until very recently. There were four alicorns in the beginning, which enabled Celestia to alicornify Twilight, but the Princesses of Magic and Love were somehow cut off from the reincarnation cycle, thus vacating their spots for Cadence and Twilight, respectively.

I've got a history of Sombra, the Crystal Empire, and Cadence's predecessor in the works myself. It's called Shattered Crystal, and... I dunno. It's sort of an unusual spin on the whole thing, I guess. Doesn't touch on the founding, though. I'm going to do a short story on that once I finish this one.