A Final Smash (最後の切りふだ Saigo no Kiri Fuda, Final Trump Card) is a special attack in Super Smash Bros. Brawl and in Super Smash Bros. 4. Final Smashes are incredibly powerful in comparison to most other attacks, generally leaving the user invincible for the move's duration, are generally unblockable, and, if properly executed, usually have the capacity to KO at least one opponent. Final Smashes performed correctly will give the player a great advantage, and may help to catch up to others or cement a commanding lead. Many Final Smashes also temporarily slow down stage elements, such as platform movements and motions in the background.
To perform a Final Smash, a player must first either destroy a Smash Ball or fall sufficiently far behind that they are given a "Pity Final Smash". Once either of these happen, the player's character will be engulfed in a multi-colored aura; pressing the special move button in this state will trigger the Final Smash. However, if the player's character takes sufficient damage after collecting a Smash Ball and before performing the Final Smash, the Smash Ball will fly out of their character and be returned to play.
Only one Smash Ball or Final Smash can be present on-screen at any given time.
Prior to Brawl
It was revealed in an interview with Nintendo's president, Satoru Iwata, that Masahiro Sakurai had intended for Final Smashes to be included in the original Super Smash Bros., even going as far as to record voice clips for each of the Final Smashes. The idea, however, was held off until Brawl. While the voice clips cannot be listened to in ordinary gameplay, the debug menu allows the player to listen to various clips that do suggest the idea, such as Ness yelling "PK Starstorm!".
Pity Final Smash
A Pity Final Smash occurs when a player is KO'd and is 5 points behind the current leader (thereby allowing the player in 2nd place or below to earn one, provided the gap is large enough.) This player respawns with the ability to perform a Final Smash. This Final Smash cannot be dropped by the player. Pity Final Smashes can be disabled only by turning the Smash Ball off, simply setting overall item frequency to 'None' does not prevent players from getting them if Smash Balls are still turned on.
While Final Smashes tend to be unique among characters, there are certain categories they can be sorted into.
In competitive play
In tournaments, all items are turned to off and none, including Smash Balls. This means that Final Smashes do not appear in tournaments. While there has been some dispute in the past whether Smash Balls should be the only item turned on, due to resembling the "super move" concept of more traditional fighting games, the general consensus is that Final Smashes are heavily detrimental to competitive play. Just like with other items, the appearance and spawn spot of a Smash Ball is still random, which can lead to a player gaining a large advantage from just being in the right place at the right time. Most Final Smashes are also considered very broken, with many guaranteeing the loss of a stock in competent hands, if not just heavy damage. The immense power of Final Smashes is also exacerbated by the frequent appearance of Smash Balls (which will spawn about every 30 to 45 seconds even with the spawn rate set to low), which heavily centralizes the match on "getting the Smash Ball" rather than outplaying the opponent. Finally, the loading time required before a Final Smash actually becomes available to a character is somewhat random and invisible to players, so one may end up using a neutral special for no apparent reason and possibly providing the opponent a free opportunity to counter.
Because items are turned to off and none, and tournament rules specify only 3 stock (or sometimes even less), Pity Final Smashes also do not appear in tournaments. One might argue that allowing Pity Final Smashes would allow a badly beaten player to stage a comeback, as they do not appear at random like Smash Balls do. However, this can only happen in a match of at least 6 stocks, which is considered to be way too high for tournaments (taking a very long amount of time to complete matches), and competitive players generally consider an element designed solely to aid a worse-performing player to be anti-competitive and thus detrimental to competitive play.
For Super Smash Bros. 4, to appeal to hardcore players and accommodate competitive play, Sakurai has stated that Smash Balls (and thus Final Smashes) will remain togglable. As a result, barring extreme mechanic changes, Final Smashes will continue to not be a factor in competitive play.