This is the secret quest of G.U. Vol.2! Once again by doing these you'll encounter 4 monsters not found anywhere else in the game, ditto for the rare armor and accessory you'll obtain! Like before, things kick off with certain posts on the boards.
Keep an eye out for the following posts as you play through the game since they'll lead you into the secret quest! The dungeons these posts direct you to are super short. Just a straight line through a few rooms until you reach some dialogue and then the mini-boss battles. No other baddies are even there!
- The World
Thread: A hidden character discovered?
Reply With: Area Words?
Thread: What's the deal?
Reply With: Which dungeon?
When these are done, and you've beaten the main story of the game, you'll get an e-mail letting you know a new quest has popped up at the quest shop!
Invasion of the Super Beings
Party Members: Any
Rewards: Flickering Scale, Book of Power, Book of Stamina, Text of Power
As the game tells you when the battle starts, The One Sin has a shield which switches between Light and Dark elements and that using the opposite element skills is the way to go. So make sure to at least have Haseo able to use Lei Zas and Ani Zot or bring lotsa Light Arrows and Air Stroke scrolls! You can break through these weak points with physical attacks... but it's gonna take much, much longer and really suck! Especially because once the shield is down and the weak point exposed, it's only a matter of time before the shield goes back up.
The fight is very doable with a party in the high 80's, low 90's level range. Even a party at level 100 is unlikely to defeat it before the shield returns. So again, if you're relying on physical and not magical attacks this fight will drag! Probably best to bring party members with high HP and let them or Haseo take care of healing with items, just be sure to stock them up with some! Also giving them a Boost Candle and equipping it to their accessories to double the healing item effects is a good idea!
Afterwards you'll get an e-mail about the "Return of the One Sin" event which is now unlocked. This isn't a proper quest as you don't start it from the quest shop. You're given the area words to Dark Morrigu Barrow Wall where the God Eater is waiting. Its a much much stronger version of The One Sin, not just in HP but also its attacks power, but the way you fight it is largely the same... though with much more healing! Plus your team should be closer to L100 than not as going in with the same levels you defeated The One Sin with is just asking for a game over! The reward for beating this baddie is the Rainbow Scale armor!