"This is the mark of an assassin, a Shadowhunter. The hidden blade. I have two. One for each wrist. The earliest models needed a sacrifice of cutting your ring finger off. I use the Da Vinci model, so I don't need to cut my ring fingers off."
In Silence and in Shadow: An Assassin's Handbook - Assassins are more than blades for hire. Indeed, they are more than just "goth rogues." An assassin is a determined and dangerous individual. In order to become an assassin, one must first pledge part of their soul to the Shadowfell. That in and of itself speaks volumes about the assassin.