What if a machine could stop serial killers-before their first murder? And what if the man who built it was already losing his mind? Kharkiv, Ukraine, 2010. Grief-stricken and spiraling into psychosis, academician Kamenev has nearly completed the "Syringe"-a machine that can send a human back in time. His purpose is personal: to prevent the brutal murder of his beloved wife, Lida. But night after night, Kamenev is haunted by a figure from his youth- Baalphégor, a demon who tempts inventors and visionaries.Baalphégor offers a glimpse into a twisted future: Kamenev's invention will outlive him and be used to exterminate serial killers across post-Soviet nations, before their crimes ever begin. The executioners-first Ostap, then a young man named Luka-grow into dark reflections of the evil they hunt. Luka's descent into apathy and cruelty raises an unbearable question: Can you kill for justice without becoming what you hate?The Demon Within is Part I of the Stone Experiment Trilogy: a philosophical sci-fi thriller where trauma, time travel, and morality collide.