There Can Be Only One
Sanctuary on Holy Ground
No Immortal may kill or take another Immortal's head on consecrated ground, regardless of the antiquity or origin of the consecration. There are only two possible recorded instances of an immortal breaking this rule: Pompeii, A.D. 79 - immediately before the eruption of Mt. Vesuvius, and the Watcher Sanctuary in 2004, when Jacob Kell slaughtered the Immortals gathered there, which may suggest the Sanctuary was not actually sanctified. Immortals may experience/witness warning signs when they are about to break the rule - such as in the fight between Connor MacLeod and the warrior Kane in 1994. According to Juan Sánchez Villa-Lobos Ramírez from Highlander, this rule is a tradition with the force of law.
One on One
Immortals must fight one to one. No two or more immortals may attack another simultaneously, or interfere with an immortal duel that is under way. While another tradition with the force of law, this rule has apparently no real consequence, saving that only one Immortal will receive any Quickening. It is, therefore, the rule most often bent or broken by immortals - for examples see Xavier St. Cloud, Jacob Kell, and Jacob Kell's Gang. Amanda also broke this rule, killing Zachery Blaine after he was disarmed by MacLeod.
Immortals also have the following commonly understood and accepted rules and customs:
- They must keep their challenges and affairs secret from the mortal world.
- Immortals do not fight or discuss their business in public. The only exception is within relationships, as immortals sometimes share the entire truth of their existence with their significant other.
- Immortals who "die" in public are expected to leave the area, in order to avoid contact with those who knew them.
- New/young immortals are assumed a brief "grace period" in which they receive training and protection from an elder immortal. This is ignored occasionally, as shown by the actions of Damon Case.
In addition, many immortals bind their own codes of ethics within the Rules, often issuing their challenges against those immortals who have offended their personal morals in some way or fashion.