After defeating Corbenik at the end of the game this quest is unlocked in the post game clear data. This quest can be started by going to the theta server quest shop, indicated by the Invasion of the super beings email.
After you get your team assembled and accept the quest your next desitination is a dungeon accessed by the " theta clouding invincible huge being" area words. A titan dead mini-boss awaits you at the beginning of the dungeon and is pretty much a push over. After this you meander your way through the dungeon to be warped away to Morrigu barrow after some cut scenes with some familiar minions.
At the wall you will encounter the one sin who will come from the cliff side to engage you in a fight. This battle involved dropping a defensive shield by attacking the two arms in order to open up the core where damage could be inflicted. The only thing which posed a threat was the beam attack he launches from his core when he is ready to re-raise the shield around his body after a certain period of time. After you conquer the boss he will give you the flickering scale to accompany the status increasing items you get from the quest shop back in town as awards.