Find Mii is based on the final section of the 3DS’ game/app StreetPass Mii Plaza. I never fiddled with that on my 3DS so I never understood the reference until I looked it up. It’s also new to me for Ultimate as it was only in the 3DS version of Smash 4. I had it on WiiU.
The god of the stage is the Dark Emperor. This dragon-ghost god can not only hurt you, but he can increase your stats, or even make sections of the stage disappear.
Which makes it a stage that disrupts the flow of many people’s strategies. But I like that in a good way.
I also like how your own Miis will appear in the captured prince/princess. While far from my favorite stage, I do like it.