The following is an excerpt from The Watchers Guidebook, pages 22-23:

ABOUT IMMORTALS

What the Watchers know about Immortality and The Game is based upon thousands of years of observation and chronicling. While we may never know all the rules of the Game, here is what we know or can conjecture.

Where do Immortals come from?

We do not know where Immortals come from or what determines who is Immortal. What we do know is:

• Immortals carry the seed of their Immortality within them through their mortal life.
• Immortals do not discover their Immortality or manifest any symptoms of it, like their rapid healing ability, until after their First Death.
• There is nothing one Immortal can do to confer Immortality to a mortal, no matter how badly they may want to.
• We have no recorded evidence of any Immortal having natural parents. Therefore, we conjecture that they are all foundlings.
• At the time of their First Death, Immortals cease aging. If an Immortal is 35 at the time of his First Death, he will appear to be 35 forever. Likewise, if an Immortal is 10 or 70 at the time of First Death, he will appear as 10 or 70 forever.
• It appears that, under certain circumstances, it is possible for an Immortal to recognize a latent Immortal who has not yet experienced their First Death.

What are Immortals like?

Like mortals, Immortals come in all shapes and sizes, good and evil. But they do have a number of traits in common.

• Immortals can only be killed by beheading.
• Immortals are not superhuman. They do not possess super strength, hearing, vision, or any similar "superheroic" abilities.
• They get hungry, thirsty, and tired just like mortals. They need sleep and they feel pain.
• They can catch colds -- they just cannot die from them.
• They appear to die, at least temporarily, from any injury which would kill a mortal.
• They do, however, have remarkable healing abilities.
• Immortals can sense one another's presence. However, apparently they cannot differentiate one Immortal from another this way.
• Immortals appear to be sterile. There is no record of an Immortal male or female conceiving a child, either before their First Death or after.
• Seasoned Immortals often become mentors to new Immortals. Through this mentoring relationship, new Immortals usually learn how to fight and the rules of the Game.

What are the rules of Immortal combat?

Immortal combat is governed by the following seemingly unbreakable rules:

• There is no fighting on Holy Ground.
• This rule seems to be immutable. It also appears to apply to harming mortals. Although we don't know the exact definition of Holy Ground, it seems to include religious structures like churches or temples as well as sacred places like Indian burial grounds. The only fight engaged on Holy Ground reported in the Chronicles took place in Pompeii in 79 A.D., and it is possible this flagrant disregard for the Holy Ground rule may have been punished by the eruption of Mount Vesuvius, which destroyed the city.
• No two on one.
• Even the vilest Immortals who have been instructed in the Game will not engage battle as part of a team. NOTE: There have been rare instances of untrained Immortals attacking in numbers, and so this rule seems more of a rule of honor than an immutable law of nature.
• No interference once battle is joined.
• While Immortals have been known to interfere or even take over a challenge before combat is engaged or after combat has ended, no Immortal can directly interfere in a combat already in progress. NOTE: See above rule and corresponding notation.

What is the Quickening?

Here's all we know about Quickenings:

• The victor of an Immortal battle receives their opponents power through a mystical process known as the Quickening.
• It is unclear to the Watchers exactly what transfers during a Quickening. Some theorize it is strength, knowledge or life experience, but quite frankly, we just do not know.
• Witnessing a Quickening is similar to watching a major electrical storm -- windows have been observed to explode, lights short circuit. It is almost as if the victorious Immortal is in the center of a lightning storm. For your safety, the Watchers recommend witnessing a Quickening from no closer than a distance of 50 yards.
• It is not possible for a mortal Watcher to become Immortal by witnessing a Quickening. There is also no danger to unborn children should the mother witness a Quickening.
• If a mortal beheads an Immortal with no other Immortal present in the immediate vicinity, the Immortal will die but there is no observable Quickening. So far as the Watchers know, that Immortal's power is lost forever. It is known that if an Immortal is beheaded by a mortal and another Immortal is present, then a Quickening will occur to go to the nearest Immortal. The Watchers do not know if there are range limitations to this effect.
• When Immortals are beheaded by inanimate objects -- guillotines, boat propellers, locomotives, or in some type of accident -- then the same rules apply as listed above.
• An Immortal cannot refuse a Quickening, even if that Immortal does not wish to receive it for any reason.