Devils are alive, and walk among us. This has been known to the human civilization ever since the gates to hell opened up, releasing the Devils upon our world. Although the gate was sealed and the Devils vanquished, it has weakened the barrier between worlds. This allows strong or oppurtunistic devils to cross over through "holes" that periodically open up as spirits to possess humans.
When a devil takes over a human, they become passengers in their own body, but are still aware of the internal war that is going on. There are certain changes that become notable while under possession. Their pupils glow red, strength and speed increase, and their endurance increases, allowing them to take numerous punishment without slowing down or being killed. There are three ways in which a human can be freed. First, the individual is killed, sending the devil back to hell. Second, the spirit leaves willingly, which they sometimes do when they are backed into a corner. Finally, a devil can be excorcised from the body through a dangerous, ancient excorcism that can potentially kill the victim.
I am a Pheonix Wright, a defense attorney hired to represent a serial killer who was possessed by a devil and forced to commit horrible crimes. The devil left his body after it had its fun and knew it was about to be caught. The individual is being charged with eleven homicides, which normally would not be a big deal. However, he is also being linked to a gruesome crime of kicking two puppies while jaywalking, a despicable act which has made this a death penalty case.
I cannot exactly bring the demon into a court room for cross-examination. Naturally nobody is going to just take the defendant's word for it that he was not in control over his body at the time of his actions. How can I go about proving that my defendant was possessed during his crimes and that "a devil made him do it"?