When did the story of the ultimate first-class Rex come to light?

At first, his identity as Rex was exposed. But Dadong still regards Rex as a friend, and Rex blames himself. He gave the hand of Ares to Dadong for safekeeping and left alone.

Finally, Rex appeared again, but it was a corpse. Rex was slapped in the back by the black dragon, so he was controlled by the black dragon. The black dragon asked Rex to kill Dadong, so Rex was in pain. Rex told Dadong to be careful when he was awake, saying that he was sent by the Black Dragon to kill Dadong. Rex was actually a good man.