You need 65 gems to unlock this level.
Captain Toad finds himself in an area full of shadows. Pick up a Potted Piranha Plant. Follow the shadows towards the screen for a 1-up. Up ahead are Fire Piranhas. Have the Potted Piranha Plant defeat them for you. Go through the door to enter the shadows. Here, you'll find Fuzzies and a giant Piranha Plant. Go through the door to exit the shadows. Pick up another Potted Piranha Plant and use it to defeat the pink enemies that fit into the wall. On the other side is a Gold Mushroom. Keep going and walk behind a wall to find the Green Star near Bowser's shadow.
Special Challenge: Collect the Gold Mushroom.