A kingmaker scenario, in a game of three or more players, is an endgame situation where a player unable to win

Lame duck (game design)

A lame duck, in the context of a game, is a player who remains in the game but has no chance of winning. It must be literally impossible— not merely highly improbable — for a player to win for it to constitute a lame-duck scenario...

 has the capacity to determine which player among others is the winner. Said player is referred to as the kingmaker

Kingmaker

Kingmaker is a term originally applied to the activities of Richard Neville, 16th Earl of Warwick — "Warwick the Kingmaker" — during the Wars of the Roses in England. The term has come to be applied more generally to a person or group that has great influence in a royal or political succession,...

or spoiler. No longer playing for him- or herself, he or she may make game decisions to favor a player who played more favorably (to him or her) earlier in the game. Except in games where interpersonal politics, by design, play a decisive role, this is undesirable.

In the context of a democratic election

Election

An election is a formal decision-making process by which a population chooses an individual to hold public office. Elections have been the usual mechanism by which modern representative democracy operates since the 17th century. Elections may fill offices in the legislature, sometimes in the...

, this scenario is termed the spoiler effect

Spoiler effect

The spoiler effect describes the effect a minor party candidate with little chance of winning has in a close election, when that candidate's presence in the election draws votes from a major candidate similar to them, thereby causing a candidate dissimilar to them to win the election...

.

Gladiator example

Consider this simple game: Three gladiators play, with strengths 3, 4, 5. In turn, each gladiator must engage another, and they begin combat. The result of combat is that the weaker player is eliminated, and the stronger player loses strength equal to that of the weaker player. (For example, if "5" attacks "3", "3" will die and "5" will have strength 2.) The winning gladiator is the last one standing.

Each round of combat eliminates one gladiator, so there will be two rounds of combat. The first round of combat will eliminate one participant and weaken the other to a strength no greater than 2. The nonparticipant's strength is at least 3, so he is guaranteed to win the second round of combat, and the entire contest. Therefore, the game collapses: The winning gladiator is the one not involved in the first battle.

Hence, the gladiator whose turn comes first is the kingmaker. He must be involved in the first battle, hence cannot win, but with the liberty of choosing his opponent in that battle, can elect either of the other two players to be the winner of the contest.

Kingmaker scenarios in practice

Because they allow the outcome of the game to be determined by a player of (presumably) inferior strategy, kingmaker scenarios are usually considered undesirable, though to some extent they may be unavoidable in strategy games. Of course the argument can be made that this means the winner, chosen by the kingmaker, played with the additional restriction of not annoying the other players as much, presumably a more difficult task. In these games, the game mechanic

Game mechanic

Game mechanics are constructs of rules intended to produce an enjoyable game or gameplay. All games use mechanics; however, theories and styles differ as to their ultimate importance to the game...

s, players' outcomes and strategies are often so interconnected that to eliminate all possibilities of this situation is almost impossible.

In tournament situations where for instance the first few teams proceed to the next round, a player that is already guaranteed to proceed can experience a situation similar to a kingmaker.
He can sometimes influence who of the remaining players comes in second (when 2 players proceed). For such a player it can be profitable to make sure the weakest player proceeds, because this reduces his competition in subsequent rounds.
This is often seen as undesirable because it conflicts with the concept that the strongest few are allowed to proceed to the next round.

Different games deal with the kingmaker problem in different ways:

• In the game of Puerto Rico
  Puerto Rico (game)
  Puerto Rico is a German board game designed by Andreas Seyfarth, and published in 2002 by Alea in German, by Rio Grande Games in English and by Κάισσα in Greek....
  
  , players conceal their victory point totals. This makes it unclear (unless players are especially attentive) which player is in the lead.

• By introducing randomness, games can make sure that everyone in the game still has a chance of winning the game, no matter how bad their situation may be. Obscuring victory points can also accomplish this, for not knowing whether one can still win can have the same effect as still having a chance to win. For example, in the group stages of tournaments such as the FIFA world cup
  FIFA World Cup
  The FIFA World Cup, often simply the World Cup, is an international association football competition contested by the senior men's national teams of the members of Fédération Internationale de Football Association , the sport's global governing body...
  
  , the final set of games in the group are often played at the same time. This results in fewer situations where one team has no chance of proceeding.

• Games can have rules that eliminate players who have no chance of winning the game, so they can not influence the game further (poker
  Poker
  Poker is a family of card games that share betting rules and usually hand rankings. Poker games differ in how the cards are dealt, how hands may be formed, whether the high or low hand wins the pot in a showdown , limits on bet sizes, and how many rounds of betting are allowed.In most modern poker...
  
   and Risk
  Risk (game)
  Risk is a strategic board game, produced by Parker Brothers . It was invented by French film director Albert Lamorisse and originally released in 1957 as La Conquête du Monde in France. Risk is a turn-based game for two to six players...
  
   are examples of this).

• Games, especially in a tournament setting, can attribute value to places other than the first place. The potential kingmaker would be able to play for his own benefit even if he can't win anymore. In the example of the gladiators, if the gladiator with strength 4 has to be involved in the first round, he will choose the gladiator with strength 3 as opponent instead of being indifferent.

• Most games prohibit—with penalties greater than match loss (for example, ostracism, disqualification)—questionable or unsportsmanlike conduct
  Unsportsmanlike conduct
  Unsportsmanlike conduct is a foul or offense in many sports that is not necessarily a violation of the respective sport's rules of play, but violates the sport's generally accepted rules of sportsmanship and/or participant conduct...
  
   geared toward effecting a kingmaker scenario.
  • Stalling, or intentionally slowing game play in timed games, for personal advantage or to that of a currently leading player, is normally treated as unsportsmanlike conduct.
  • The use of revoke
    Revoke
    To annul by withdrawing.In trick-taking card games, a revoke is a violation of important rules regarding the play of tricks serious enough to render the round invalid...
    
    s, or intentional rules violations, in trick-taking card games
    Trick-taking game
    A trick-taking game is a card game or tile-based game in which play centers on a series of finite rounds or units of play, called tricks. The object of such games then may be closely tied to the number of tricks taken, as in plain-trick games such as Whist, Contract Bridge, Napoleon, Rowboat, and...
    
    , to void a round and effect a kingmaker scenario is discouraged by use of severe penalties. In tournaments, doing this can be classified as cheating.

• In the later rounds of a Magic: The Gathering
  Magic: The Gathering
  Magic: The Gathering , also known as Magic, is the first collectible trading card game created by mathematics professor Richard Garfield and introduced in 1993 by Wizards of the Coast. Magic continues to thrive, with approximately twelve million players as of 2011...
  
   tournament, the rules permit a player to concede defeat for any reason other than bribery. Therefore, even if a player is too weak to obtain one of the top eight win-loss records, such that s/he cannot advance to the three-round single-elimination playoff that concludes the tournament, s/he can wield some degree of kingmaker influence by a) conceding to the one or more players whose victory s/he would most prefer but b) doing his/her best to obtain a win against anyone else. Especially in tournaments featuring lower numbers of players and rounds, such that both a) players are more likely to encounter friends/allies and b) each win or loss counts more heavily, kingmaker situations are therefore abundant but difficult to avoid. For this reason, many players follow a practice of conceding to anyone whose deck clearly outclasses theirs and who has a strong chance of winning one of the eight playoff spots, thereby avoiding as far as possible even the appearance of engaging in unsportsmanlike collusion
  Collusion
  Collusion is an agreement between two or more persons, sometimes illegal and therefore secretive, to limit open competition by deceiving, misleading, or defrauding others of their legal rights, or to obtain an objective forbidden by law typically by defrauding or gaining an unfair advantage...
  
  with any party in preference over any other.