The Game, is a millennia-long forced conflict between Immortals, experienced as a series of elimination duels fought free-form between Immortals. Combat continues, influenced by The Gathering, until there is only one survivor, hence the Immortal mantra, "There can be only one!"

Any Immortal may challenge any other Immortal at any time, so long as the challenged Immortal is not on Holy Ground at the time, or are in a populated area. Challenges must be in private, as witnesses are a danger. To issue a challenge, an Immortal should approach their target and announce themselves and their intention to fight. Combat may then begin immediately, utilizing any weapon besides a bladed weapon, however, is a breach of the established rules.

While this is the standard form for a challenge, there have been occasions, and Immortals, that skirt or disregard the rules entirely. Incidents in which one Immortal shot an opponent and fled, or enlisted mortal mercenaries to bring down an opponent and beheaded the "Dead" immortal, or hunting with a group against a lone Immortal, and other violations or borderline violations, while rare, have occurred.

In Highlander: The Series, The Messenger was an Immortal of unknown origin whose message was peace and the abandonment of "The Game," but his mission ended in failure and his own death.