Mistakes
The blistering heat cooking Nero's neck was more than enough to drive a man to his knees, kicking and screaming. In fact, Nero was not immune to it, and even wanted to cover his neck in his blue flame to reduce the temperature. But he knew that his neck wasn't actually hot. Rather, the nerves in his body were being hijacked, being made to feel the pain and suffering as if he was being burned.
As morbid as it seemed, his father had taught him about such torture techniques a long time ago, as well as how to identify them. It was a fun trivia game they played as kids. Moreover, one of the reasons he was silent for so long, listening to the masked man's drivel, was because he was waiting to see if his tattoos reacted to it. Indeed, just as he had expected, his tattoos slowly moved from his back towards the mark on his neck. That was enough to give him the confidence that he needed.