A Final Smash (最後の切りふだ, Last Trump Card) is a type of special move in Super Smash Bros. Brawl and in Super Smash Bros. 4. Final Smashes tend to be incredibly powerful attacks, often leaving the user invincible for the move's duration (the periods vary from character to character, but always apply upon startup/execution), 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 in a match that they are given a "Pity Final Smash" upon respawning. Once either of these happen, the screen will darken, the player's character will be engulfed in a multi-colored aura and their eyes will glow a bright yellow; in this state, the character's neutral special move is effectively replaced with a single use of their Final Smash.
In Brawl, the majority of Final Smashes are powerful enough they have the potential to KO incredibly early or even one-hit 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) randomly 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. Additionally, Pity Final Smashes cannot occur while the Smash Ball is present on the battlefield, even when the Pity Final Smash requirements are met.
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 "none" and "off," respectively, meaning that Final Smashes do not appear in tournaments. While there has been some dispute in the past whether Smash Balls solely of all items should be allowed, due to Final Smashes resembling the "super move" concept of more traditional fighting games, the general consensus is that they are detrimental to competitive play for several reasons:
Because items are turned to off and none, and tournament rules generally specify around 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.