The mythology of Highlander is based on the premise of the existence of Immortals. Immortals are both born and made, a latent immortal will live and age normally should no violence befall them during their lives, but should a potential Immortal meet with a violent death, that person would awake from death, their aging process stopped, their ability to heal increased. Without a violent death, however, to trigger Immortality, the potential Immortal ages normally and dies a natural death. No Immortal, latent or active, can reproduce, they are sterile.
To become stronger, they must absorbQuickening, and there fore they enter into The Game, the systematic dueling of one another. The Quickening can even double their knowledge of combat. The only way an Immortal can die, is to be beheaded.
Fitz falls to Kalas
Should an Immortal lose his or her head, an energy release is called the Quickening and infuses the nearby Immortal. The last Immortal survivor inherits the totality of all the Quickenings, called The Prize. When there are only a few Immortals left, they will feel drawn to a particular location where they will fight to the last. There are few rules: an Immortal is safe only on Holy Ground, and only single combat is allowed. If an Immortal is beheaded and there is no other Immortal nearby to absorb the Quickening, that Quickening is lost.
The Quickening can repair almost all damage to the body, without scarring, except death by decapitation. It cannot regenerate lost limbs, but can regenerate organs and reattach limbs, as long as the Immortal is in the condition to do so.