Shadow Mario Paint
When activated, Bowser Jr. will transform into Shadow Mario, and then paint an orange "X" on the screen. Anyone caught in the "X"'s hitbox will take damage. Bowser Jr. will then be teleported above the top camera boundary and will be able to move around freely while the "X" is onscreen. The "X" deals knockback each time a character walks into it, and the knockback usually sends them towards the middle of the "X", making it difficult to escape. When the attack ends the "X" explodes, dealing relatively high knockback. All hits taken from the paint have the same knockback regardless of damage until just before the "X" clears away.
Bowser Jr. disguised himself as Shadow Mario using the Magic Paintbrush, which was invented by Professor E. Gadd (who also invented various other machines, including the Flash Liquidizer Ultra Dousing Device, the Poltergust 3000, the Poltergust 5000 and the Poltergust G-00). The Magic Paintbrush was either given to or stolen by Bowser Jr. prior to the events of Super Mario Sunshine. In said game, the paint used by Shadow Mario was dependent on the level in which he appeared. The glowing orange used by Shadow Mario Paint bears a close resemblance to the paint used in Pianta Village. Paint in this level behaved similarly to lava, dealing damage and causing Mario to jump after taking a hit, similarly to how it works in the Final Smash. It, along with the Junior Clown Car, has become a recurring theme for Bowser Jr., especially throughout many spin-off titles. The Koopalings use the Magic Paintbrush to transform into Shadow Mario as they are Bowser Jr.'s alternate costumes, but they have never used it or their Junior Clown Cars outside of Smash.
Much like Metal Mario, Shadow Mario has become a separate character from both Bowser Jr. and Mario. He appeared as a playable character in Mario Golf: Toadstool Tour. Despite using the paintbrush in spin-offs, Bowser Jr. does not tend to turn into his alter ego.
Shadow Mario Paint's name refers to Mario Paint, one of the few games to utilize the SNES mouse.