From Hero to Demon King

Gavin Cole is summoned to another world, forced to become the Hero and slay the Demon King. He is betrayed by the world and becomes the new Demon King. Trapped for twenty years, he holds onto the song 'Riding the Waves,' sung by his Mother. On the verge of death, he triggers a reverse teleportation, returning to his hometown to reunite with his Mother, who has waited twenty years for him.