Emperor's Stand-in: My physique improves a little bit every day

Emperor Daning was possessed by a demon and needed to be in seclusion for three years to heal his wounds. Lu Changjing, who traveled through time, became his substitute and had three palaces, six courtyards and seventy-two concubines.
However, Eunuch Cao was eyeing him covetously and threatened Lu Changjing that he could only look but not use him, otherwise he would be killed without mercy.
Fortunately, when he traveled through time, he merged with a special object and obtained an immortal demon body, and his physical fitness could increase by one point every day.
The physical fitness of an ordinary person is one point, and Lu Changjing can increase the physical fitness of an ordinary person every day.
Physical fitness determines life span, defense and endurance, and can affect strength, speed, reaction, spirit... and even talent!
The immortal demon body has various characteristics and can be awakened and upgraded continuously, such as: strengthening the body, hiding, deep roots, night vision, endless life... until rebirth by dripping blood, Pangu power, transcending time and space, immortality!
Three years later, Emperor Daning came out of seclusion and was surprised to find that all the civil and military officials in the court, the three palaces and six courtyards, the seventy-two concubines, the huge empire... had become the shape of Lu Changjing.