Super Guide Block
From the Super Mario Wiki
Ads keep the MarioWiki independent and free :)
A Super Guide Block is a special green ! Block that appears in the single player modes of New Super Mario Bros. Wii and New Super Mario Bros. U. It will appear when the player has lost eight lives in New Super Mario Bros. Wii (five lives in New Super Mario Bros. U) consecutively in the same level. If Mario hits the block, the Super Guide mode will be activated, Luigi will appear and the computer within the game controls him to clear the stage in place of the player. If the player presses on , the player may choose to deactivate the Super Guide mode and take control of Luigi. However, if the level is beaten, it does not count as an actual win. Items collected do not count as well.
This feature returns in New Super Mario Bros. U Deluxe, acting identically to its appearance in the original Wii U game. However, when playing as Luigi in New Super Mario Bros. U, the block is colored red and summons a computer-controlled Mario in place of Luigi.
Note that if a Super Guide block appears in New Super Mario Bros. Wii, stars on the save file will no longer be shiny. However, in New Super Mario Bros. U, stars on the save file will stay shiny as long as the Super Guide is not used.