Truth serum!
It was a military drug created seven hundred years ago by 'Demonic Doctor' Robert Terson. It was distilled from dozens of hallucinogens and psychedelics. Upon intake, it would cause a person's mind to reach a 'calm state' that was no different from numbness.
Under the 'calm state', the person's subjective reasoning would be repressed and their subconscious and unconscious thoughts would become the main driver. One would respond in the most truthful manner in response to any voices from the external world. There would be no room to hide any secret hidden deep in their heart.
