Prologue/Introduction:

In this world Shadowhunters have a Naming Ceremony that is the final protection against demonic influence. After Sebastian Verlac, nobody can be too careful when it comes to safety in the mind. The young Nefilim will also receive the "sacred Mark" from a Silent Brother that they choose. This Mark will stay on them forever, show them as one of the Angel's warriors, and will even tie bits of the dead ashes together. It will represent who they are, and if it ever fades, the Shadowhunter can't get a new one, and they will be vulnerable.

In a sense, it's the last protection when everything else fails.