How to get this quest
This quest is available in Nephlune's Adventurers' guild after completing Rise to Intermediacy and Blue Sugar Menace.
Reaching the source of the phantoms
Your target is in Nephlune's vault. The vault's entry is in the underground, guarded by a lamia. Once inside the vault, you need to:
Defeating the phantom lord
The phantom lord is resistant to physical attacks and has a very high MDEF. Furthermore, this enemy is not a demon (meaning you can't exploit Light II or The Art of Light).
Aside from removing Aura's collar, there are two ways to effectively damage the phantom lord:
Once the boss is defeated, reporting to the guild is the last step.