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Smash Controller is an application downloadable off the Nintendo eShop that allows players to use their Nintendo 3DS handhelds as a controller in Super Smash Bros. for Wii U, without owning Super Smash Bros. for Nintendo 3DS. The app was released on June 14, 2015, and is a standalone version of the Wii U connectivity menu in Super Smash Bros. for Nintendo 3DS.
Nintendo eShop / Nintendo website blurb
This application is intended for use by consumers who own the Super Smash Bros. for Wii U game and want to expand their controller options by using a Nintendo 3DS family system as a controller—even without owning the handheld version of the game. This application includes the same capability that is included in the Super Smash Bros. for Nintendo 3DS game. Simply purchase and download the Smash Controller application from the Nintendo eShop to your Nintendo 3DS system and get ready for battle!