The Darkness comic book has been around since 1996, but you don't need to worry about fifteen years of continuity in order to understand 2K Games' The Darkness II. You just need three minutes with Johnny Powell.
Powell here gives us the low-down on The Darkness, from the beginning of time up to the death of Jackie Estacado's girlfriend and beyond. That death, depicted in The Darkness' first video game outing, remains one of the most heartbreaking moments in gaming.
You really shouldn't go into the sequel without giving the first a go, if only to experience that moment. You can find it used at most video game shops for a couple of bucks, and you've got plenty of time to catch up.