You must pause the game first and use the Directional Pad to enter these codes. Successful entries will sound off he words of "Necro Goddess" or "Necrolicious":

All Weapons: Up, Down, Left, Right, Left, Right, Left, Left.

Infinite Ammo: Up, Down, Left, Right, Up, Left, Down, Right.

Infinite Health/Necroplasm: Up, Down, Left, Right, Right, Left, Down, Up.

Infinite Necroplasm: Up, Down, Left, Right, Down, Left, Up, Right.

No Blood: Up, Down, Left, Right, Up, Up, Up, Up.

Open All of the Comics: Up, Down, Left, Right, Right, Left, Left, Up.

Open all Missions: Up, Down, Left, Right, Left, Left, Right, Right.

Open the Encyclopedia: Up, Down, Left, Right, Left, Right, Up, Down.