The Lucky Bell is a new power-up in Super Mario 3D World that transforms Mario, Luigi, Toad, Princess Peach, or Rosalina into their their Lucky Cat forms. It is called a Lucky Bell because when the character Ground Pounds, they will transform into a gold cat statue of themselves and a trail of Coins is made as they fall. It also grants them temporary invincibility for as long as the player holds one of the Ground Pound buttons, which is similar to the Tanooki Suit from Super Mario Bros. 3, and the Statue Leaf from Super Mario 3D Land.
The Lucky Bell looks somewhat like a cross between a Mushroom and a Power Bell, which fits how its first appearance is in World Mushroom.