The Smash Ball is an item found in Super Smash Bros. Brawl and Super Smash Bros. for 3DS and Wii U. It will only appear once in a battle, and when one of the characters obtains it, they are able to unleash a final smash on any opposing fighter. Smash Balls can be switched on and off like regular items, and they don't appear when a character has collected all the Dragoon pieces.
"A mighty item. Each character has a unique secret technique known as a Final Smash that can only be used when that character acquires a Smash Ball. Use it well, and you'll be one huge step closer to victory. It can turn the battle's tide, help you put the finishing touches on a complete thrashing, or even provide a chaotic finish that's sure to leave you laughing until you cry."