Mystery Codex is Eden Studios' first sourcebook for WitchCraft. It is largely a rule book, with some character information, like new associations, and even some detail on the default WitchCraft universe.

This being the game's first supplement, it includes notes on how to update the first edition of WitchCraft, originally published by Myrmidon Press, for use with this and future supplements.

• Character types: the immaterial Spirits and Undead, bodies risen from the grave.
  
• Creation rules and powers for haunting Ghosts, the sensation-junkie Phantasms, Essence-draining Vampyres and the Relentless Dead, reborn through sheer rage or willpower.
  
• Associations: Pariahs who lurk in the underbelly of society; Iscariots, devoted to making restitution for their crimes; the monster-hunting martial artists, the Storm Dragons; and the Thanatoi, preoccupied with death and existence thereafter.
  
• Metaphysics: more invocations and necromantic powers, the body-warping Disciplines of the Flesh and the Storm Dragons' Tao-Chi.
  
• A brief tour of some of Geburah, the Sephiroth of the dead.