Smashdown (全員バトル, Everyone Battle) is a multiplayer mode in Super Smash Bros. Ultimate. It consists of a series of matches, and after a character has been used in a match that character is not able to be chosen again until the series of matches is completed. It is one of three modes in the game's Special Smash mode, the other two being Custom Smash and Super Sudden Death.
Each player chooses a different character and participates in a standard Smash battle. After the battle ends, the characters used disappear from the roster for the rest of the game. Mii Fighters are not available in this mode and, just like Squad Strike, Echo Fighters are always displayed and counted as a separate character from their parent, even if the player choose to merge. Also, the number of battles played before the game ends can be adjusted, and enabling Mercy Rule will cause the game to end if a player has won enough battles to make losing impossible. Replays can't be saved in this mode.
In competitive play
While not as popular as other modes, Smashdown can occasionally be seen as a side event at tournaments. Due to how the mode is structured, a traditional bracket is often seen as impractical, so a version of the round robin style is used where the player(s) with the most wins by the end is declared champion. Typically only 2-8 players are placed in their own tournament for easier logistics, and they play each other until the game declares a winner. This means multiple of these micro-tournaments can be active at once, and a grand finals of sorts can later be played with all of the winners. The mercy rule is usually turned on, but this is at the discretion of the tournament director.
Fighters are usually picked on the spot, which allows for some unique mind games. If a player knows the main of their opponent, they can pick that fighter early on to eliminate them from later fights. This strategy may not necessarily benefit the player in the short-term, but it will hurt their opponent in the long-term. Also, players could intentionally mislead their opponents by picking characters they have little experience with to put them under a false sense of confidence by easily defeating them with their mains early on, only for the tables to turn later on by the player using their main against the opponent with a fighter they have little experience with. However, this will inevitably create situations where multiple players want the same fighter, causing a dispute that could slow down the tournament. The most common solution is for players to claim fighters ahead of time.