From the Super Mario Wiki, the Mario encyclopedia
Morton's Hammer is Morton Koopa Jr.'s signature weapon in New Super Mario Bros. U, New Super Luigi U, New Super Mario Bros. U Deluxe, and Paper Mario: Color Splash. It is a variant of the hammer seemingly filled with magic that Morton uses to knock Mega Pokey segments towards the player. When Morton swings at the Mega Pokey, a Pokey segment is launched into the air or along the ground, while the last segment and head are launched at the same time. After the entire Mega Pokey has disappeared, Morton becomes incapacitated, making him vulnerable to Mario's jumps. The hammer replaces Morton's wand from the other Super Mario games.
In Paper Mario: Color Splash, while not the same hammer as the one in New Super Mario Bros. U, the wooden hammer transformed from his magic wand that Morton wields has the same shape. Eventually, the hammer is set ablaze, and if the Fire Extinguisher Thing card is used, the hammer is reduced to just its handle. Morton then hits Mario with the stick to attack.