The Darkness II, like a lot of games nowadays, 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.
There are currently no known push button cheats for this game but our exclusive guide http://www.supercheats.com/guides/the-darkness-2 will more than help you navigate your way through. This sequel takes place two years after the events of the first game, players are once again put back in control of Jackie Estacado. The guide includes finding ALL the relics and an Achievement guide.