There are 29 Relics of the Darkness that are spread out through the single player campaign that will nab you the Relic Hunter Achievement/trophy. Each of the relics that you discover have a long history behind them that are read to you by Jonny and explain just what each of the relics purpose was in history.
You do receive some extra essence for each of the relics you find, but the backstory alone is worth the trouble of searching for them. If you have the Limited edition of the game you can purchase and equip the relic hunter ability which makes finding them a bit easier, but not nearly as easy as we have made it for you!