You're the Shadow Man, a person who can switch between the world of the dead and of the living. In Shadow Man your mission is to prevent 5 serial killers from bringing the total armageddon. In the world of the living, you're mortal and have conventional weapons. When you die, you are transported to the world of the dead and you become Shadow Man once again. Which means you're not really mortal. In the world of the dead you can't use normal weapons, your gun shoots ghosts which kill the zombies and the other ugly creatures which live there. You also have different voodoo powers and weapons. And you're carrying a large inventory around.
Go to Asylum:Gateway and jump down onto the square pipe and turn left. Go past the flame buckets(with Gad Toucher) and eventually after passing some pig guards you should come across a room with alot of boxes stacked on one side and a cage with a Deadsider in it. Jump up the boxes and get to the top. Then jump onto the beam supporting the arch and do so again to the next and in the middle of the last one you should unlock play as Deadsider! Then go out through the other side and some Dark Souls lie in wait for you.
Flame Grilled
Go to the last room in the Temple of Life. There are five altars that let you extend your life meter. Run up to the second from the right to activate this strange cheat.