The Master Ball works very similarly to the Poké Ball, in that it summons a random Pokémon when thrown. However, unlike the Poké Ball, the Master Ball is almost guaranteed to summon a Legendary of Mythical Pokémon. Though these Pokémon also appear from Poké Balls, the Master Ball is far more likely to summon them.
The Master Ball is a variety of Poké Ball introduced in generation I, and arguably the rarest of all Poké Ball variants. Developed by Silph Co., it is unique in the aspect that it is guaranteed to catch any wild Pokémon that it is thrown at without failing, which makes it popular for catching Legendary Pokémon due to their low catch rates. Normally, only one can be legitimately obtained per save file, without recurring to other game elements such as hacking or winning one by luck on the Pokémon lottery.
When it was first implemented, this item had the unique function of opening up and temporarily capturing nearby opponents when thrown. This will be changed in Beta 1.2 with the inclusion of the Trophy Stand, which functions very similarly.
The concept of an opponent-catching Poké Ball was first pitched in the early stages of SSF2's development in the form of a Premier Ball, another Poké Ball variant. Whether the Master Ball's initial concept was inspired from this idea or if it is merely coincidence is yet to be disproved.
The Master Ball is currently the only unlockable item in Super Smash Flash 2.
In event #36: Gotta Cherish Them All, the event required to unlock the item, the Cherish Ball is used instead of the Master Ball. This item is functionally identical to the Master Ball's original use, except it only opens upon hitting opponents, and after capturing one it ignores stage collisions and passes through platforms, resulting in one-hit KOs.