Are there any good fics out there about pre-show Discord? When he was originally banished by the Princesses, his origins, etc?

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I do have one, but it's pretty silly. It's called 'Hello, and My Name is sir Real'.

Thanks, I’ll take a look<3

https://www.fimfiction.net/story/11495/the-steadfast-sky

It's a classic.

Have one planned, but it would be a doorstopper.

alarajrogers is really good at Discord stories. She is slowly working on a big long backstory for him, with future stuff she'd like to add later. You may have to wait to get the whole story but ALL of her fanfiction are good reads.
It doesn't help that she has soooo many ideas, so she is always making new stuff.