The way Viktor rewrote those people's memories was like this. The prophecy said when the human king will summon the heroes, the demon lord will come to Erok. The demon lord will not be someone from the human race but he will be somehow related to the demons. Means, he will be a mixed race between human and another demonic race but not from a pure-blood demon. But he will possess a pure demonic aura purer than any demon can possess.
Viktor erased those memories where he met Keya as the demon lord. In that memory gap, he recreated a moment where he promised Keya he'd introduce her to the demon lord because he's related to him by blood. The demon lord is his brother's only son and his mother was a half demon. She died after she gave birth to her son. Also, the demon lord's biological father who was a human had died even before he was born.
