From Kirbypedia

English Name:	Rainbow Sword
Japanese Name:	虹の剣 Nijino Turugi Rainbow Sword
Game:	Kirby's Dream Land 2
Boss Used Against:	Dark Matter
Anime Episode(s)	None

A legendary item that Kirby's wields to battle Dark Matter. Also considered a copy ability, though he doesn't need to inhale it to gain its power.

Games

The blade is powerful weapon needed to exorcise the evil from King Dedede in Kirby's Dream Land 2. Without it, the day cannot truly be saved. It is unlocked by collecting the seven Rainbow Drops scattered across the seven Rainbow Islands.

The weapon can only be used in the final battle against Dark Matter. As with many of the ultimate items, Kirby can freely fly while it is held even without inflating. Unlike many of the final abilities, the sword does not produce a projectile when it is swung by Kirby.

Rather, it is slashed down and up to rebound attacks made by the final boss. These reflected shots inflict heavy damage upon Dark Matter, far more than striking with the sword deals.

In Dark Matter's first form, the sword wielding warrior, the energy balls it throws from its sword can be sent back. These split into three pieces when they are struck: one is angled straight ahead, one angled upward, and one downward. This way, no matter where the attack was delivered from, it can return to Dark Matter to damage it.

In its second "true" form, the orbs fired from Dark Matter's back can be reflected back. These do not splinter and are simply hurtled back at it; however, they are delivered in a higher volume, giving more opportunities to return fire.

Kirby Copy Abilities:

Cupid - Animal - Backdrop - Ball - Balloon - Baton Twirling - Beam - Bomb - Bubble - Burning - Clean - Cook - Copy - Crash - Cutter - Fighter - Fire - Freeze - Galaxia - Ghost - Hammer - Hi-Jump - Ice - Iron - Jet - Laser - Light - Magic - Metal - Mike - Minimum - Missile - Mirror - Needle - Ninja - Paint - Parasol - Plasma - Sleep - Smash - Smash Bros. Abilities - Spark - Stone - Suplex - Sword - Throw - Top - Tornado - UFO - Water - Wheel - Wing - Yo-yo

Final Weapons:

Star Rod - Love-Love Stick - Rainbow Sword - Star Chariot - Master - Triple Star - Block