Mysterious Forest is a battle stage that's new to Mario Kart: Mega Mash-Up.
This stage is a large map in which there is a central area of flat grassland in which the players start off in. They can then either use some of the item boxes and battle here, or head of into the forest with surrounds the grassland. There are many entrances to the forest around the central area and these all wind around in a maze fashion. There are also various item boxes dotted around the forest area allowing for combat in that area.