Why Zeldris has the power of the demon king? In the manga it's mentioned that Zeldris has the power of the demong king. And I believe in the anime too. What does this mean? Why Zeldris has the power of the demong king?
Demon King is ruler of Purgatory. 3000 years back, during the fighting their bodies were destroyed and in order to retrieve his body. So he grant Zeldris the power of demon king but in reality he is making Zeldris a container for his vessel to grow his body.