I was reading a story where a guy becomes a changeling "king" over a thousand years before the show.

Flip I forgot what the name was

Think your looking for Twisted Fate by Reykan
(That's the only one that comes to my head)

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https://www.fimfiction.net/story/187595/the-king-is-dead-long-live-the-emperor

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I think this was it

It's been a couple months and I've only read the first 3 chapters out of 10-12
(Thought I'd bookmarked it at the time)

It was Twisted Fate, Thank you.
And this other one "The King Is Dead, Long Live The Emperor!" looks to be good too.
Will Read!