The mythology of Highlander is based around 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 the that death, their aging process stopped, their ability to heal increased. Without a violent death the potential Immortal ages normally and dies a natural death.  No immortal, latent or active, can reproduce.

To become stronger they must absorb Quickening.  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 called the Quickening infuses the nearby immortal.  Generally an immortal loses his or her head because another immortal takes it. This is called “The Game,” and will theoretically continue until only one Immortal is left.  The last 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.