In the Bible Cain killed his brother Abel. Cain was then cursed and marked by God. The Brand in this case is the "Mark of Cain" and part of Cain's burden was to walk the earth and gather warriors to defend the world from the war that's coming. All of these warriors bare the mark of Cain which grants them abilities and protections against the supernatural.
UPDATE: The preview issue was just released! Check it out!