Any immortal may challenge any other immortal at any time, so long as the challenged immortal is not on holy ground at the time. To issue a challenge, an immortal must approach his/her target and announce himself or herself and their intention to fight. Combat may then begin immediately. Utilizing any weapon beside a bladed weapon skirts the rules.
While this is the standard form for a challenge there have been occasions and immortals that disregard or skirt the rules entirely. Incidents where 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.
The Messenger was an immortal of unknown origin whose message was peace and the abandonment of the game.