|“||And the Lord said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the Lord set a Mark upon Cain, lest any finding him should kill him.||”|
–From the Bible, King James Version Genesis 4:15, City of Ashes
The Mark of Cain, sometimes referred to as the Mark of the Wanderer, is a protection mark meant to protect the bearer from anyone with the intent to harm him. The Mark is among the ancient, angelic runes that is not in the Gray Book and thus should not be used by Shadowhunters.
Anyone who attempts to hurt anyone with the Mark on them will be punished sevenfold. When the bearer of the Mark is attacked, the Mark burns white and the assailant is attacked by a being that falls from the sky with the same intent that was to be used against the bearer. While the wearer would be invulnerable to attacks from others, according to Magnus Bane, he/she can still be killed by oneself and accidentally through inanimate objects.
Despite being a protection spell, it is also seen as a curse to the bearer. Biblically, there are several variations of the curse associated with Cain and the Mark. However, the one primarily similar to Simon's is the burden of living eternally, as it prevents his or her death, and wandering the world long after his loved ones have passed. At one point, Clary speculated that the Mark of Cain acts directly on others, like an external force that turns the wearer into a Wanderer by cutting him away from his family.
According to Magnus and the Shadowhunter's Codex, the Mark of Cain is considered the first Mark, being the first Mark a human received from heaven for protection. According to Raphael Santiago, it was drawn by the Maker himself.
In 2007, in preparation for the Mortal War, the vampire Simon Lewis had Clary Fray, using her gift with runes, draw the Mark on him as protection against Raphael Santiago and his clan, who meant to kill Simon and whom Simon meant to trick into participating in the war. Without time to think of consider other options, Clary agreed and placed the Mark on his forehead.
Weeks after, Simon was attacked by men under the orders of the Greater Demon Lilith. His mark burned white, and the assailant was struck down by a heavenly force that reduced the man to a pile of salt while leaving Simon completely unharmed. Some time later, Lilith wanted to harm Clary during a confrontation; however Simon stood in between them and Lilith was destroyed by his Mark of Cain.
Shortly after, while facing Angel Raziel, the angel was infuriated and appalled at the sight of the Mark on a mortal, even more so that it was on a vampire, applied by a hand other than God's. The Angel would have killed the boy, had it not been for the Mark's effect. In exchange for his life and the loaning of the sword Glorious, Raziel ultimately stripped the boy of the cursed Mark.