Mushroom Kingdom U is another new stage in Smash 4, based off of New Super Mario Bros U. Rather than being based off of just one area of NSMBU, Mushroom Kingdom U is an agalamation of all of it. Well, some of it.
So this here stage starts off in Acorn Plains, but it's got icicles and stuff. Then Kamek shows up and turns it into different areas, like the Rock-Candy Mines, Meringue Clouds, and a Castle. Sounds like a good stage, right? However, this stage is not without it's faults. Nabbit's in this stage, and he can insta-kill anybody unless another person attacks Nabbit. In other words, unless you're in a team battle, you're fucked.