Nintendo has dedicated this year to Mario's brother Luigi and in honor of the green plumber is releasing his own stand alone downloadable pack, Super Luigi U DLC, for New Super Mario Bros. U. The digital download is set to release in both NA and EU on June 20 and a stand alone package on August 25 for NA and July 26 for EU. The digital download will cost $19.99 and $29.99 for retail, you won't need a copy of New Super Mario Bros U to play the packaged title.
In addition to a release date and price Nintendo Direct showcased Nabbit, the creature the player chased in the original game, as a playable character for Luigi's adventure. He'll be available for multiplayer games and won't take damage when touched by enemies but cannot use power-ups.
Source: Nintendo Direct