This is a 6 person BC fight against Zeid. Only characters that are currently doing the mission, or have previously done the mission (regardless of current allegiance), can enter the BCNM.
Once inside the Throne Room, run straight across to the big door on the other side. Heal up, buff, eat food, and then when you are ready, enter the BC by targeting the door and selecting "Where two paths converge".
Buffs are removed upon entry.
Do not move towards Zeid until everyone is inside.
Eventually (when Zeid is at about 65% hp) Volker will join the fight to help you and after a short cutscene, the battle will resume. Volker will attack Zeid with you and must be kept alive. If he is killed the battle will end in a loss.
Zeid will spawn 2 mobs called Shadow of Rage which have decent attack but very low hp. Blizzard III or stronger will one-shot. They are very resistant or immune to Sleep and Lullaby.
If the shadows are defeated the boss can just call another two.
After the fight watch another cutscene and then return to the president's office and talk to Iron Eater.
Watch another cutscene and finish the mission.
An adventurer has been attacked while on an investigation to Xarcabard -- and the attacker is said to be none other than the Dark Knight, Zeid! Report to the President's Office for a briefing on the situation at hand.