That's a bad translation with tie-ins between Erdrick and the Dragon Lord made up. Here is a translation of the actual DQ1 manual (taken from Dustin's DQ Shrine):
Of course, Radatome = Tantegel, Laura = Gwaelin, Roto = Erdrick, and King Dragon = Dragon Lord.
Keep in mind that the story in the manual says something contradictory to the game, that the Dragon Lord is a man. When he transforms into a dragon, it says "The Dragon Lord shows his true form!" So what's in the manual is probably just meant to be the general understanding of the Dragon Lord, and not the full story.