Since the stage was in the 3DS version of Smash 4, it’s new to me in Ultimate. It’s based on an area in the Pokemon: Black and White games. I didn’t play those either so I don’t really know the true area on which its based.
The stage is that typically standard quo in design. Except for when the pokemon show up. Zekrom is the most notable.
For he’ll shoot a orb of pure energy at the side of the stage. Which can hurt, so stay away from it.
But it will tilt the stage, which make things a little different but nothing too drastic.
And Reshiram is more on attacking you with fire moves. It’s an okay stage, but far from my favorite honestly.