A bargain-bin game whose bargain bin presence is entirely unjustified.
The titular Darkness is a mysterious force that requires a human host to survive, and it awakens on its host's 21st birthday. This occurs as your Uncle Paulie sends out a hit on you (its the mafia, things like that happen). So, as Jackie, you have two things to worry about over the progression of the game: Avoid Paulie's thugs, and avoid succumbing to the increasingly powerful Darkness.
The Darkness, anchored to you, requires darkness to function and allow you to use its powers. Usually you have to stick to shadows but often you can easily shoot out lights to create darkness of your own. The Darkness can be seen on the sides of your first-person view as two demonic eel-like creatures anchored from your back. You can devour the hearts of your dead enemies via these eels to increase your Darkness level. They definitely feel like creepy, living creatures as they fight over victims' hearts, look around, or growl.
You can use the powers of The Darkness to fight Paulie's minions along with your regular arsenal of guns, and they are very helpful and very fun to use. The first power is the Creeping Dark ability which allows you to control one of your eels and slither along the ground, up walls, across ceilings, etc. They are generally more troublesome than helpful during combat and they will most often be used for the purpose of solving simple puzzles. Your second Darkness ability is the Demon Arm. It is a tentacle that shoots out and impales your enemies, and can also be used as a way to destroy light sources. You can do it from further away with guns, but using your guns depletes your ammo. The third ability is Darkness Guns. They are two guns powered by the dark that zap enemies and lights or send enemies flying with some kind of physical boom. The final, and coolest, Darkness power, is Black Hole. It is fairly self-explanatory, although it is slightly unsatisfying as the damage done doesn't match that of an actual black hole.
It sucks up nearby enemies and physical objects, but, strangely enough, doesn't actually make them disappear into the black hole. When the bodies plop back onto the ground after the hold fades away, its a bit disappointing that they aren't sent to some unknown realm. (This thought made me realize how well this game taps the player's lust for death and destruction)
About a quarter of the game takes place in the Otherworld, a mysterious and hellish realm that is just terrible. Everywhere you look its depressing and intimidating, like something out of a death metal music video.
For much of the game the music is very subtle, but there are a few select parts where it shines as fantastic. The story of the game is extremely interesting and draws characters which you will actually care about. The voice acting across the entire enormous cast is excellent.
Which leads me to this question: Why is this a bargain bin has-been? This game is great in every respect. It's a shame that it isn't as popular as it deserves to be. But you, as a consumer, are lucky enough to take advantage of this. Do yourself a favor and buy this game for five dollars. It will be one of the best five dollars you ever spend.