@Luca H.
Having put dozens of hours into the game, and having long since given up on it, for a number of reasons, the part with the Dragon is more-or-less the tutorial stage, given how it introduces a number of the game’s concepts, mechanics and such.
After you defeat him in the main story questline, a dungeon appears that’s essentially a crystallized memory, where you get to fight him again once a week for a chance of getting some pretty interesting loot, depending on your level and the level of the dungeon you select.