The official theme of this wedding is Dungeons & Dragons! The recent D&D movie (characters pictured above) might be a good reference to watch if this is all totally new to you.
If you're already familiar with D&D, feel free to do basically whatever you want. No real weapons with your outfit though, not even peace bound.
If you are NOT familiar with D&D, it includes concepts seen in medieval fantasy like The Lord of the Rings-- elves, wizards, knights, that sort of thing. Costumes and outfits that give that vibe would fit right in.
We are aware that we've invited "normal" friends and family to our special day as well. ;p Our first suggestion as an alternative to D&D would be anything you might wear or dress as to a renaissance festival. Bards, fairies, vikings, you name it. It might be a good idea to check your local ren fest's website for their vendor list so you can do online shopping that way. Otherwise there are plenty of folks on Etsy who make attire that fits this theme.
Our second backup suggestion is clothes that lean toward bohemian; flowy and floral can work.
And of course, if all else fails, we'd rather have you THERE dressed like you're at a normal wedding than not have you at all, so please do that if it's most comfortable to you!