Highlander

Highlander is a casual constructed variant of Magic which allows only one copy of each card in the deck, with the exception of basic lands. The name Highlander is a reference to the movie of the same name, whose tag line was "There can be only one".

Rules

 * Any card from Alpha up through the latest Magic set can be used (except for banned cards). Cards from the Portal sets, International Collector Editions, and World Championship Decks can be used provided you put all your cards in opaque sleeves and the International Collector Edition card edges are rounded.
 * Special mulligan rules: In some variants, the first time a player takes a mulligan, they draw a new hand of as many cards as they had before. Subsequent hands decrease by one card as normal. In other variants, a mulligan of 7-6-6-5-5... is used (i.e. a second mulligan of each size except 7).

Individual playgroups and regional variants often have their own house rules regarding many aspects of the game to suit their own style and to make the game more fun. This often includes a more extensive banned list, or banning certain combinations (e.g. decks can include a Thopter Foundry or a Sword of the Meek, but not both) or modifying how interactions work (e.g. infinite loops only recur a set number of times to prevent infinite damage or mana loops that would otherwise win the game immediately) or replacing the banned list with a "points" list. In these cases the objective is to lengthen the game and prevent good draws from being able to win or knock a player out of the game too soon. Given how big the card pool is, there are many plausible, if unlikely given the format, ways to deal infinite damage or otherwise win within a few turns and this is generally considered to not be in the spirit of the game, and can be enforced with house rules if the group feels they need to.

Deck construction

 * A deck may not contain two cards with the same English name except basic land cards.
 * In many variants, such as Canadian Highlander (see below) or European Highlander, a deck must contain at least 100 cards. In Australian Highlander, decks follow standard magic construction limits (60 card main decks, 15 card sideboards)
 * Some cards are entirely banned in tournament formats. Since these are non-DCI-tournaments, they have individual ban lists.
 * No sideboards are used.

Singleton
In the simplified Singleton version of Highlander players can use Standard, Modern, Legacy or Vintage deckbuilding rules. A deck has no 100 card minimum.

Australian
Australian Highlander is a primarily paper-based format that uses the vintage banned list and has a special twist on deck building. It has a point list for some of the more ubiquitous pieces of power and combo pieces available to deck builders. You only get to use seven points in your deck of sixty cards. Australian point-system highlander was first played in late 1996 or early 1997.

Canadian
Canadian Highlander or Canlander is a paper- and online Vintage format with a unique twist on deckbuilding. It features its own banned list and has a point list for some of the more ubiquitous pieces of power and combo pieces available to deck builders. You only get to use ten points in your hundred-card deck.

Points List

European
European Highlander (also known as German Highlander) is a 100-card, Singleton, 20-life, best-of-3 Vintage-based format without sideboards. Unlike other popular variants, European Highlander makes use of a banlist instead of a points system. The format "watchlist" is maintained by the council, and visible on the European Highlander website. It uses the "London mulligan".

Historic Singleton
MTG Arena has once released an event known as Super-Singleton, which has a similar community format known as "Gladiator". The decks are 100-card, singleton, 20-life, with a card pool of the Historic format (aka the Arena card pool). As of yet, the ban list is of two cards, Oko, Thief of Crowns and either Nexus of Fate (for Super-Singleton) or Field of the Dead (in Gladiator). As the format is not otherwise official (and exist as Direct Challenge only), the game parameters are up for debate, but generally accepted as "best of 3, no sideboards".

Commander
Commander, formerly known as Elder Dragon Highlander (usually abbreviated to EDH) is a Highlander variant format with specific rules centered around a legendary creature designated as a player's "commander".

On the less serious end of the spectrum, there is a Commander drinking game that can be played, where every time you cast your commander you have to finish a drink.