Mark of Cain
The Mark of Cain is the source of the First Blades power, it was placed on Cain by Lucifer after the murder of Abel. The mark and the blade work together, and without it the First Blade is rendered useless. The mark can also be transferred, if the person is worthy of it i.e. a killer. According to Cain, taking on the mark comes with a great cost and burden.