Let's summarize the story so far.
Three races lived in peace. Giants are physically strong, dragons are powerful with their magic, and humans have special abilities.
Due to the giant's curiosity, the dragons taught them magic. Then the humans got interested and learned magic as well from the dragons and improving their strength thanks to the giants. That's how Warrior class and Magician class first started.
Humans who only had special abilities that are totally random could become superhuman with the strength from the giants and magic from the dragons. Though they were still far weaker than them.
Then the giants, using magic, locked the dragon away here, in Amazon Forest in Brazil because they wanted to conquer the world. But they didn't know that the Dragon King would lock them away in another dimension.
