The Smash Ball (スマッシュボール, Sumasshubōru) is an item in Super Smash Bros. Brawl. It allows the user to activate a powerful Final Smash attack when it breaks. It only appears in Training mode, certain Events, standard and Special Brawls, not the main one-player/co-op modes such as Adventure Mode: The Subspace Emissary.
To use the Smash Ball, the characters must attack the Smash Ball until it breaks. A "fresh" Smash Ball can take around 43% damage before breaking, while a Smash Ball that's been on screen a while is easier to break. The screen darkens, and the player whose attack broke the Smash Ball goes into "standby mode," in which the character's eyes turn yellow and body begins to glow a rainbow color. The Final Smash can then be unleashed at any time by pressing the input for a neutral special move. After using a Final Smash, the character is no longer in standby mode and must break another Smash Ball before using it again. If a character takes enough damage while in standby mode, there is a chance that they may lose the Smash Ball, which then begins to move about the screen again. Any projectile, including Poké Balls and Assist Trophies, can aid in collecting it.
When a Smash Ball appears, the screen blinks and the crowd gasps. The Smash Ball then moves around on its own, making it harder for players to use; it must be attacked by the players until it breaks, and whoever breaks it will be able to use their Final Smash. As an item, it can be turned off in the Item Switch option for those who wish to battle without the potential imbalance that comes from such powerful attacks.
The apparent physics of the Smash Ball tend to vary. While it will often bounce off of walls and hard platforms, it will occasionally pass through them. The amount of knockback it takes also varies; sometimes it will take an attack without even changing its trajectory, and other times an attack will send it halfway across a stage. The camera treats Smash Balls the same way that it treats characters. Smash Balls do not vanish by being forced off the stage like other items, so a Smash Ball sent off the screen can pull the camera away from the characters.
If no one breaks the Smash Ball after a long enough period of time, it flies away. However, the longer it is flying around, the easier it is to break. It can also be damaged by the stage (for example, Norfair's lava), and if nobody strikes it before it is destroyed, it is lost. Smash Balls are also affected by glancing blows; the Smash Ball will not take damage but will be pushed away. When a character is in standby mode, no Smash Balls appear until the Final Smash is used (or the Smash Ball is lost and flies away). This means that only one Final Smash can be used at once.
 Description from the Super Smash Bros. Brawl instruction booklet
Destroy it and unleash a Final Smash with B.
This item is also available as a trophy, with random availability. It is a fairly common trophy.