You start to develop a demonic alter-ego. This mutation can be taken in several steps, with each step counting as one mutation. This is a potentially very severe limitation, worth up to four picks.
1. Split Personality:
You develop a Dr. Jekyl and Mr Hyde-type psychosis. There is a voice in your head; it cannot force you to do anything, but it can withhold up to half or your Attributes Fu, Fortune and Magic points each session and use these to force concessions from you. This second personality has access to all of your memories and abilities, and will actively try to set up situations where it can entrap and corrupt you further.
2. Sleeping Shadow
Your shadow will occasionally possess you. This will be at times when your normal self is out of control; asleep, unconscious, drugged or just very tired or absent-minded. When your shadow is in control, it will continue to behave as before, by creating sticky situations that cause you trouble or lead you further into depravity. You might suddenly find yourself a molester or murderer in you sleeping hours.
3. Awakened Shadow:
Your demonic shadow can now possess you at any time, at the GM's whim. Such possession is limited to about one-tenth of your waking time. As dictated by the needs of the story, the demon will seize control of you in the middle of normal game sessions.
4. Shadow Separation
The shadow can now separate entirely from you, as a Demon of a power corresponding to your own, or slightly more powerful. It is still very much focused on you, and as long as you are alive, it does not suffer from SupernaturalCreature.
This is often the first step towards defeating the shadow; as it gains separate physical existence, it is much easier for your ego to accept and face it down.