Story related and pops upon Completing the "The Demon Fort of the Yellow Heaven" main mission. Imo the boss was actually the easiest out of the first handful of missions that I did, although that might be due to extra morale gained due to an invasion that I won right before. Some tips:
- There is a nice farming point in the courtyard before the final Battle Flag if you'd like to max out your morale before tackling the fight but Imo anything 21+ is basically all you'd need.
- What helped me out during the fight was staying behind him once my companions aggroed him as it seemed like many of his attacks (like the lightning storm) are variants that are mostly frontal attacks.
- You can double summon by this point of the story so i'd recommend getting either someone IRL or an extra AI to help buff up your DPS while ensuring that aggro your way gets minimized.
Below is a video of the fight (all credit to WitnessHQ):