Cronus became the Demon King when the previous one died at the hands of Aegeus, and his friend became the mighty hero of Arcadia. Cronus's hate towards the humans got worse when people suddenly attacked his fellow demon kin out of nowhere. Many of his kind died, and the humans got away with only shallow cuts and bruises on their bodies.
The Demon King, who was residing inside Fritz's mind, was the same Demon King in the memories playing before their eyes. Cronus eventually married a demon woman that was Esecleus' real mother. She was the same woman he saw in his memories.