Dark Souls not dark enough for you? Try this.
Via PC Gamer, Pure Darkness is a mod for the PC version of the game that takes the Tomb of the Giants' lighting effects and applies them to the entire game.
Made by Notaprofi, it might seem as though it makes the game - especially boss fights - impossible to beat, but he says they "are proven to be beatable, and the areas - passable. May be with some luck, but mostly by skill."
Which sounds like business as usual, really.
You can download the mod here.