The Darkness II features a New Game + mode. This allows you to play through the game for a second time, or any time after that, with your character intact from the previous play. You can use this mode to get the remaining relics that you couldn't get on your first playthrough.
To unlock it, complete the game once on any difficulty.
We have an exclusive guide http://www.supercheats.com/guides/the-darkness-2 for this sequel that takes place two years after the events of the first game. There are currently no known push button cheats for this game but our guide will more than help you navigate your way through. The guide includes finding ALL the relics and an Achievement guide.