The Rainbow Sword is a weapon created by collecting all the Rainbow Drops. Kirby uses it to fight Dark Matter in Kirby's Dream Land 2. It doesn't fire projectiles unlike most other Final Weapons (However, it can reflect Dark Matter's attacks). Despite its name, it's only black and white, due to graphic limitations on the Game Boy, albeit it displays in multicolor, during the ending credits, using the Super Game Boy.
Kirby swings the Rainbow Sword, chopping enemies. This ability can launch back Dark Matter's projectiles.