From the Super Mario Wiki
The Warp Star is a common item in the Kirby series. It first appears in the Mario series as an item in Super Smash Bros. Melee and later appears in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS / Wii U. When a character grabs one, they will fly up into the air, and then crash down to the ground, making an explosion that damages other players (except the player who used the Warp Star).
A Warp Star also appears in the Subspace Emissary mode of Super Smash Bros. Brawl. Kirby and Peach/Zelda use it to escape from the stadium right before it is swallowed by Subspace. However, they are chased by the Halberd, and are forced to land on it.
Super Smash Bros. Melee
Super Smash Bros. Brawl
Super Smash Bros. for Nintendo 3DS / Wii U