Bab 2172: "Never To Return" 

Back then, the abyss demon king should not be able to leave the abyss to fall in love with the princess of the elf and family, but because of the king's specialness, he could escape from the abyss once in a while. 

Unlike other demon kings and queens who only thought of conquering the main world, this king was more interested in developing the abyss! 

So, even if he could escape from the abyss, the king was not interested in destroying the world or anything, which was why this king was not widely known by the experts from the world union. 

The previous demon king just fell in love with the elf princess and gave birth to Axelle, who was then left alone at the Elven Continent because both parents couldn't stay there anymore. 

Axelle didn't have any concept of having parents because his parents should have left him behind right after he was born, which was why he grew up with other elves as an 'oddity'.