Perhaps because of the Magic Power Virus, Roy was now very sensitive to the fluctuations of magic power.
When he entered the underground laboratory and saw César's body in the culture tank, Roy noticed that there was no trace of magic power in this body.
In terms of time, even if César had found a woman of the solar eclipse and then gave birth to a half-human half-demon hybrid, this demon-human body would still need more than ten years to grow into its current state. However, when César was in the Abyss, he had abandoned his original body and placed his soul in the Balotan Pillar to return to this world. It was impossible for him to spend more than ten years waiting for this body to grow slowly, so this demon-human body was forced to mature through black technology.