Mark of Cain

The Mark of Cain (印のケイン, Shirushi no Kein) is a Curse or a special seal branded on all of the male members of the Higure Clan by Reidō; also known as the Curse of Cain, this seal is responsible for reincarnating a deceased Higure.

Overview
This curse prevents them from truly dying as long as their brain remains intact. On dying, the seal activates; healing and reviving the damaged bodies of those who bear this mark. The only way to kill someone with this mark would be by destroying their brain and nervous system. Later on, after fully awakening the Mark of Cain, Shin was able to discover that the Mark of Cain simply gives his body the ability to heal itself and regenerate from any wound by feeding off of the users's magic; that is, if he does not have enough magic, the seal will not be activated. The only reason that the it is necessary for the brain to survive is- the brain basically commands the rest of the body to regenerate. With his brain gone or destroyed, the body cannot heal. By actively using the power of the Mark, Shin is able to achieve nigh-immortality.

History
Several centuries ago, Reidō cursed the male members of the Higure Clan's with this mark because of their behavior, cursing them to be unable to find peace even in death; making them nigh immortal and unable to die. The curse was supposed to slowly drive the members insane by restoring their bodies after they have died and since this curse only restores one's body and not one's soul, they might lose their sanity if killed a certain number of times. Describing it's effects as the worst case of PTSD and eternal solitude, the curse soon became popular, so much so that mercenaries and assassins would attack the Higure clan on a regular basis, just to extract this curse.

Trivia

 * The curse of Cain and the mark of Cain are phrases that originated from Genesis 4, where God declared that Cain be cursed for murdering his brother Abel.