From the Super Mario Wiki
Shadow-Play Alley, also known as World 2-3, is the ninth regular level and the third course of World 2 in Super Mario 3D World. It is an inside course with a part of it taking place in the shadows. It is unlocked by completing Conkdor Canyon, and its own completion unlocks Really Rolling Hills. Shadow-Play Alley also appears in the Wii U version of Captain Toad: Treasure Tracker as an extra stage if the player's Wii U has save data for Super Mario 3D World.
The course starts in an underground area with some Fuzzies and Piranha Plants. Some moving platforms and Fire Piranha Plants await, followed by the Checkpoint Flag. The next area is set in the shadows, where the player has to navigate through an area full of Fuzzies, Piranha Plants, and one Mega Piranha Plant until they reach the door leading them to another area. Here, the player must pass some enemies and moving platforms until they reach another door which leads them to the Goal Pole.
Super Mario 3D World
The player needs to use the Cat Suit to climb up the wall at the beginning of the stage to find a shadow of a 8-bit Super Luigi sprite crouching in the top left corner.
Captain Toad: Treasure Tracker
An 8-bit Luigi can be seen in one of the tunnels by moving to the right and looking inside the tunnel.
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