If you know your Zelda, you know that Demise has a mark that of which Gannon/ Gannondorf has from Twilight Princess. This allows theories to take place between the two evil kings. Demise looks greatly like a gerudo also, which means he could be one. The proof to that point: There is only one male Gerudo every hundred years, and after Demise died, Gannondorf rose up and took his place. That could be why Gannon knows that if he happens to take, even a third of the Tri Force, he will be very powerful.