The inheritance of the Demon Lord has always been a secret. In other words, the inheritance of the Demon Lord is a "curse", and the Demon Lord is the recipient of the "curse".
After the fourth demon king came to the throne, he searched for the origin and removal of the "curse" based on the unfinished words of the third demon king before his death.
Purpose: I firmly believe that justice will defeat evil. If victory comes at a price, I am willing to give my life in exchange for the safety of my loved ones.