Pause the game and enter the codes, when a code is entered correctly, you will hear "Necro goodness" or "Necro-licious"
All weapons
up,down,left,right,left,right,left,left
Infinite ammo
up,down,left,right,up,left,down,right
Infinite Necroplasm
up,down,left,right,down,left,up,right
All missions
up,down,left,right,left,left,right,right
Open all comics
up,down,left,right,right,left,left,up