The Unova Pokemon League is a location from Pokemon Black and White, and the other Pokemon League stage in Smash 4. This stage is essentially the spiritual successor to the Spear Pillar from Brawl, in the vain that it's a big, dramatic location with legendary Pokemon spectating the battle. Man, I wish they brought back the Spear Pillar. That stage was awesome. But of course 75m can come back.
Anyways, as you fight the good fight, N's Castle will rise from behind the Pokemon League. It doesn't do much, but walkoffs are added to the stage, so that's something. The real stars of the stage, though, are Reshiram and Zekrom, who fly in and watch you fight. But watching you fight isn't enough for them, so they'll use their fire/thunder attacks to damage the stage and cover it in searing flames, or... electricyness. For some reason, Milotic, Shaymin, and Whimsicott show up on this stage too. Unlike Reshiram and Zekrom, they really don't do anything. Nonetheless, it's a great stage, even if it isn't very tournament viable.