A Final Smash (最後の切りふだ, 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 (the periods vary from character to character), 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 and their eyes will glow a bright yellow; pressing the special move button in this state will trigger the Final Smash. However, as with any other item, a character with the Smash Ball collected may randomly drop it whenever damaged, which will then cause the Smash Ball to fly out of the character and return to play.
In Brawl, the majority of Final Smashes are powerful enough that they can be activated at any time and result in a KO if used properly. In SSB4, Final Smashes are universally much weaker, many of them no longer being able to KO opponents at low percentages.
Most Final Smashes cause the camera to temporarily zoom in on the user when activated, though there are some situations where this does not occur, such as with Pac-Man's Final Smash, Super Pac-Man. In Super Smash Bros. for Wii U, through a currently unconfirmed nature, there are some cases where Final Smashes that normally zoom in don't zoom in at all.
Only one Smash Ball or Final Smash can be present on-screen at any given time.
Prior to Brawl
Masahiro Sakurai revealed in an interview with former Nintendo president Satoru Iwata that he intended to include Final Smashes in the original Super Smash Bros., even going as far as to record voice clips for each of the them. The idea, however, was held off until Brawl. Some of these voice clips, which were contained in the Super Smash Bros. debug menu, were eventually and correspondingly used in Brawl.
Pity Final Smash
A Pity Final Smash (Easy Final Smash in SSB4) occurs when any player is KO'd and is 5 points behind the current leader. 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. In a 2-player timed match, since the points differ by two for each kill, it is possible to go from a lead of 4 points to 6 without crossing the Pity Final Smash line. If a player SD's, and consequently loses only one point in an advantage, then a Pity Final Smash is possible.
In competitive play
In tournaments, all items, including Smash Balls, are turned to off and none, meaning 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 location of a Smash Ball is 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 to have disproportionate power; simply using one competently will result in massive damage if not a clean KO of the opponent, accomplishing in seconds what could take several minutes for the opponent to match. 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 no matter what frequency the items are to appear), 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 far 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.