DRAGON QUEST: DAI NO DAIBOUKEN (1991)

It starts with a kid named Dai remembering a story told to him by his grandfather, the monster magician Brass, about the defeat of the Demon Lord Hadlar by the hands of a hero who, as the story progresses, is known to be Avan. Dragon Quest Dai no Daibouken Movie 01 — Description from AniList community (anilist.co).