After the defeat of the demon lord Hadlar all of the monsters were unleashed from his evil will and moved to the island of Delmurin to live in peace. Dai is the only human living on the island. Having been raised by the kindly monster Brass, Dai's dream is to grow up to be a hero. He gets to become one when Hadlar is resurrected and the previous hero, Avan, comes to train Dai to help in the battle. But Hadlar, announcing that he now works for an even more powerful demon lord, comes to kill Avan. To save his students Avan uses a Self-Sacrifice spell to attack, but is unable to defeat Hadlar. When it seems that Dai and Avan's other student Pop are doomed a mark appears on Dai's forehead and he suddenly gains super powers and is able to fend off Hadlar. The two students then go off on a journey to avenge Avan and bring peace back to the world. 
(Source: Anime News Network) 
Notes:Chapter count includes the respectively two and three chapter short stories "Derupa! Iruiru!" and "Dai Bakuhatsu!!" which were published in earlier Weekly Shonen Jumps pre-serialization and collected in the volumes.

After the defeat of the demon lord Hadlar all of the monsters were unleashed from his evil will and moved to the island of Delmurin to live in peace. Dai is the only human living on the island. Having been raised by the kindly monster Brass, Dai's dream is to grow up to be a hero. He gets to become one when Hadlar is resurrected and the previous hero, Avan, comes to train Dai to help in the battle. But Hadlar, announcing that he now works for an even more powerful demon lord, comes to kill Avan. To save his students Avan uses a Self-Sacrifice spell to attack, but is unable to defeat Hadlar. When it seems that Dai and Avan's other student Pop are doomed a mark appears on Dai's forehead and he suddenly gains super powers and is able to fend off Hadlar. The two students then go off on a journey to avenge Avan and bring peace back to the world.
 
(Source: Anime News Network)

After the defeat of the demon lord Hadlar, all of the monsters were unleashed from his evil will and moved to the island of Delmurin to live in peace. Dai is the only human living on the island. Having been raised by the kindly monster Brass, Dai's dream is to grow up to be a hero. He gets to become one when Hadlar is resurrected and the previous hero, Avan, comes to train Dai to help in the battle. But Hadlar, announcing that he now works for an even more powerful demon lord, comes to kill Avan. To save his students, Avan uses a Self-Sacrifice spell to attack, but is unable to defeat Hadlar. When it seems that Dai and Avan's other student Pop are doomed, a mark appears on Dai's forehead and he suddenly gains super powers and is able to fend off Hadlar. The two students then go off on a journey to avenge Avan and bring peace back to the world.
 
(Source: Anime News Network)

A Dragon Quest: Dai no Daibouken spin-off focused on the past of Avan. 

A manga that is essentially the story of Enix from the time it starts working with games, until its release of the series it became known for, Dragon Quest. All the characters are real Enix employees at the time. You get to see the developers as real people, who agonize over everything, like how big to make the text boxes, what should NPCs say, etc, as well as how they get along with each other.

After the liberation of Papnica, Dai was asked by a certain boy to become his apprentice. After a few days of training, he heard rumors that there was a hero who learned Avan-style techniques, so he went to meet him hoping to learn and become stronger. Dai struggled under the guidance of an unfamiliar "master'', but...!? 


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

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