There is a powerful, sentient, extradimensional electromagnetic lifeform that desires to enter our material realm, to experience humanity. This Entity is something completely beyond our ability to perceive, comprehend, or adequately label.
Its mission here on Earth requires self-awareness, mobility, and a capacity to manipulate its environment - in other words, it has to possess a biological host, preferably, one that is a member of the Earth's dominant species for ease of access to centers of social power. However the nature of its existence makes it incompatible with human biology; no biological body could support even a substantial fraction of its basic essence without being immediately destroyed. Therefore, the Entity divided itself up into smaller units of energy, which then simultaneously entered our space-time continuum at separate discreet points, bonding with otherwise normal human infants. Such a simultaneous entry into our space-time from an extradimensional source scattered the Entity's hosts widely, not only throughout space, but throughout linear time as well. Therefore, the hosts are born in different areas of the world and at different times throughout history.
These hosts are known as "Immortals." This is because the sub-Entity units (known as the "Quickening") allows them to live forever if they are first killed by something other than disease, poisoning or natural causes. Upon their 'first death', Immortals resurrect, stop aging and can only be permanently killed when they are beheaded. The Immortals are involved in an epic conflict of battle. They duel each other to the death (via decapitation), with the victor absorbing the Quickening of the defeated. This dueling is done for a legendary and unknown "Prize". According to legend they must fight each other until there's only one Immortal left. When there are only a few Immortals left, they will feel a mysterious pull to a certain place where they will fight till the last. The last one gets the Prize. This event is referred to as "The Gathering". This battle through the ages until then is called "The Game," and there are certain rules, such as only one Immortal can challenge another, and that no Immortals will fight on holy ground.
Without a violent death to trigger their Immortality, they will age and die as other humans. The Quickening will then be imbued upon another infant. The same thing happens if an Immortal is beheaded and there is no Immortal nearby to absorb the Quickening.
Immortal bodies react to wounds and diseases exactly the same way as ordinary human bodies do until their violent first death. After that, the only thing that can permanently kill them is decapitation. As a very powerful electromagnetic field, the Quickening repairs their bodies whenever it's damaged by rebuilding their flesh from scratch by stimulating the cells still there to divide far more rapidly than normal, and revives them from temporary death even after the main organs have stopped operating. It acts like a jump start on cells and the major organs to get them going again. How long it takes wounds to heal depends mostly on the individual, but also on the type of wound and location of the wound.
The Quickening speeds up the healing process to such an extent that it happens within minutes, and the process is perfect, leaving no scars or marks because the Quickening allows Immortal cells to access the genetic blueprint to a greater extent than mortal cells. The old "wounds" of an Immortal aren't healed, because they're not wounds. They're already healed, and so nothing will be done about them. If they're reopened they'll heal normally, unless some of the scar tissue is still present. But the Quickening cannot replace parts lost from the external body, meaning the Immortal body still cannot regenerate limbs or other appendages. The internal organs do get regenerated. Because they are contained inside the body cavity, the Quickening is very concentrated around them and through them. If an Immortal's heart, for example, is cut out, the Immortal will, of course, die temporarily. Then a new heart will be grown by the Quickening and he or she will revive.
Cuts and scratches on the body heal very quickly, but on the neck and head it takes longer. Deep cuts on the neck are likely to leave scars, and are slow to heal. Also the organs inside, because when the neck is cut open, slashed or gnawed or ripped, the organs inside are no longer completely surrounded by the flesh, thus impeding the process of the Quickening. This is because the Quickening is least concentrated in that part of the body. It is the area where all the neurotransmitting takes place, very near the medulla. Logically, the Quickening is less concentrated in these areas so that it doesn't interfere with the functions of the brain and spinal cord.
Another astonishing benefit of the Quickening is that it completely protects cells from free radicals and ambient radiation, preventing any oxidative and ionization damage to the DNA of an Immortal. In addition, the Quickening has an effect on certain protein structures to drastically stimulate the production of telomerase (an enzyme that prevents chromosomal degeneration), so Immortals continuously possess the enzyme in every cell in the body. This completely prevents cellular senescence and therefore gives every cell replicative immortality. Due to a perfect cellular replication and no added damage to DNA, the development of a degenerative condition or cancerous cells is impossible. The Quickening also suppresses the functions of the genetic sequences responsible for the natural metabolic processes involved in aging. As a result of these effects of the Quickening, Immortals are physically incapable of aging after their first "death".
When an Immortal is shot, the bullets are absorbed by the Quickening. When an Immortal is stabbed or skewered, of course the weapon or tool is not absorbed. Earrings and nose pins are not absorbed. This is because the Quickening must be surrounding the object, and able to flow through it. When a solid object is completely contained inside the flesh of the body, the Quickening dissolves it. If something is swallowed, it will not be dissolved. It will simply pass through the digestive system like anything else that is eaten. If something is not completely inside the body, for example, if a knife is stabbed into an Immortal's heart, it will not be able to be dissolved because part of it is still outside, therefore the flesh is unable to be completely around the object.
Sterility is one of the main characteristics of Immortals. Male Immortals are sterile because the strong concentration of the Quickening interferes with the mitochondria in the sperm cells. Mitochondria are the organelles that give cells energy. The Quickening increases their energy to such a degree that it destroys the enzyme on the head of the sperm, making them unable to penetrate an egg cell to fertilize it. When female Immortals are pre-Immortal, they menstruate just like other women. After they become Immortal, however, they don't. This is because of the spontaneous healing qualities of the Quickening. The blood and tissue lining of the womb is unable to form, because the tiny blood vessels that supply it are unable to rise to the surface and open up. Also, ovulation is impossible, because the ovaries are also affected by the Quickening. A pre-Immortal woman can't have children because the Quickening increases the mitochondrial energy in an egg cell to such a degree that any 'invading' cell (such as a sperm cell) is destroyed. Therefore, fertilization is impossible.
Immortals are, by their nature, walking powerhouses of electrical energy, their bodies permeated with it. Yet, normally, this bioelectrical energy that is their animating force is so well integrated into the day to day workings of their metabolisms as to be completely unnoticeable; they don't give off sparks when they shake hands with people, attract metal objects to their bodies, disrupt the workings of radios when they walk by, or cause any other overt electromagnetic phenomena during their normal functions. However, they do sense one another when they come within a certain range of each other, in much the same way two magnetized objects will respond to each other's presence when brought close enough together. This sense is usually described as vibrations felt in the skull. That's because the head is the most sensitive part of the body. The pull of the Quickening is also felt elsewhere.
Immortals also insist on fighting fairly long, extensive battles with archaic weaponry, which happens to primarily consist of one of the most electrically conductive materials discovered by man. They refuse to fight on holy ground - the sites of which, according to some occult scholars, may well be intersections of ley lines, places where the Earth's own powerful electromagnetic energies come together and are vastly amplified. And the explosive special effects display that accompanies the lethal resolution of every Immortal duel is electrical in nature. All of this is because Immortals are a more highly charged form of life than normal humanity and their innate bioelectricity is the basis for their Immortality.
The reason for compliance with the codes of Immortality and traditions are in two separate categories -- practicality and compulsion.
Observing an unbreakable truce on holy ground is a pragmatic restriction. Any Quickenings occurring in a ley line conjunction might well be disrupted from their normal function, becoming explosive, painful, and actively hazardous not just to the surviving Immortal but to anyone else within a certain radius. Or, on the other hand, the ley line conjunction might simply absorb the dead Immortals life essence into itself, invalidating the Quickening for the victor and setting back or even ruining the whole Gathering process. A third possibility might be that as the victorious Immortal absorbs the electrical energy of his fallen foe, the energy of the Earth itself might also flow into him, burning him to a crisp. Whatever the case, such a risk would be unacceptable not only to the Immortals themselves, but to the Entity within all of them that originated the Gathering in the first place and thus, the ban against fighting on holy ground is universally observed. Not all 'holy ground' is actually at a conjunction of ley lines. However, the stricture is one that comes down from prehistory, when probably all such places of power were indeed on these energy nodes. Immortals also sense something about holy ground that makes them feel very uneasy about the thought of fighting there, so all of them honor the taboo regardless of how otherwise unprincipled they may be. None of them understand why they feel that way, and rarely or never discuss it, simply taking it for granted.
The strict code of combat ethics that all Immortals observe is also pragmatic. Each Immortal has his or her own individual wavelength, or frequency, and prior to a successful Quickening, Immortals must attune themselves to each other. This is accomplished by the all-out exertion of face to face combat to the death. The clashing of the swords, or the occasional non-lethal strikes, conduct each warrior's electrical field to each other, and over a course of seconds or minutes, those fields begin to resonate in unison. The final death stroke acts as a catalyst, freeing the defeated warrior's electrical energy and letting it flow into the body of the victor, whose own electrical signature is now, briefly, compatible with that of his or her foe.
The entire "Game" is a compulsion, not a choice and it's a near universal compulsion among Immortals. They don't discuss it in those terms; they are not even aware of it, preferring to think that they choose to fight each other, and protect young Immortals from each other, out of respect for their own cultural traditions - but in point of fact, they really have no choice. They are compelled to behave in this way, for the most part. With a very few, singular exceptions, they can't help it.
So, it's a compulsion that the overwhelming majority of Immortals cannot resist - they must fight each other, and they must protect young Immortals, at least, for a time. However, there are Immortals who are so ruthlessly vicious that they never mentor anyone, and some who even actively seek out new Immortals to slaughter before they can become dangerous.
Divided up, the Entity lost self-awareness, and thus, no longer has the capacity to actively pursue the agenda that drew it into our world in the first place. However, electromagnetic energy is used to store information, and so each host of the Entity has certain 'lore' fed to it, in the form of subconscious programming, that causes it to behave in a manner compatible with and commensurate to the gradual reconstruction and reunification of this electromagnetic entity here, on Earth, in a biological host body capable of tolerating and supporting the Entity in its complete form.
The Gathering is designed to gradually reassemble the original Entity into one coherent being, housed in a body that will have been through incremental exposure made more and more resistant to and compatible with the Entity's powerful life energy.
However, reassembly is not the only goal of the Gathering. The Entity has a mission, and in order to perform that mission properly, it needs to prepare itself. Knowledge is power, and as each Immortal kills and absorbs the essence of other Immortals, it also adds, at least at a subconscious level, the sum totals of those Immortals' skills, talents, and life experiences. At the far end of the Gathering is not only an Immortal possessed of all the electromagnetic energy of the original Entity, but one that will contain the sum of the life experiences of the thousands of Immortals who also once walked the Earth. If indeed knowledge is power, then the Immortal who emerges at the end of the Gathering may well be the most powerful single being to ever walk the Earth.
It would thus be a waste for new Immortals to be immediately killed before they've had a chance to gather experience. In addition, the electrical life essence of an Immortal grows and amplifies itself through time; so letting a young Immortal live for a while could be the equivalent of letting a calf mature before the slaughter. The near universal desire among Immortals to guard 'young' Immortals from harm, to give them time to learn the nature of their existence and gain some experience before they place themselves in harm's way is part of an Immortal's programmed behavior.
What lies at the end of the Gathering is something beyond the purposes and desires of mere humanity. The final Immortal who emerges from the Gathering will merely be a biological lens that will focus the energy of the cosmos for a grand reshaping of humanity. The Entity's real purpose is to learn how human bodies really work so that at the end, it will simultaneously infuse the entire existent human race, making all of us Immortal, and imparting to all of us some of the wisdom and experience it has brought together over many millennia.