This quest and the short series of events leading up to it hold 4 monsters you won't find anywhere else in the game and the final reward a rare armor you won't get anywhere else! To kick it off you'll have to visit the forums.
These posts will trickle out as you progress through the game and when you properly replay to them you'll get the area words for the place you'll need to visit. Once you're replied enter The World's title screen then back out to the Desktop and the post should be updated with the info you need. Each of these areas are extremely small, like three rooms or so long. Only the first one even has monsters prior to the mini-boss encounter.
- The World
Thread: Anyone opening chests?
Reply With: Area Word
Thread: Confusing Event...
Reply With: more detail
Thread: Found an irritating mob
Reply With: What kind of monster?
With those three events out of the way, you'll get an e-mail about a new quest appearing at the quest shop!
Lord of the Dead
Party Members: Any
Rewards: Word of Vigor, Word of Spirit, Text of Power
Like the other areas leading up to the quest this one also takes place in the caves. However it's much larger and two floors deep! Unlike normal areas there aren't monsters in every room, but the ones that are there are the very same mini-bosses you defeated to get here! When you reach the dead end deep into the second level you'll be transported to Indieglut Lugh to face the big boss!
Cernunnos can cause Poison and Curse status effects very easily so having Cleansing Pill and Jinx's Bane attached to your parties armor might be a good idea. Even if not, if your party is at L50 its not a tough battle. Just have a few Healing and/or Revive Rains and you should be fine! Starting the battle with a Beast's Awakening will get things going pretty smoothly, and you'll probably end it with one too. Remember that using items on your party members helps to fill up the morale meter!
I like using Haseo's sword for this battle for the damage and because combos can be hard to pull off. If Cernunnos knocks you away, use a Skill Trigger skill to shoot right back over to him since by the time you run back, he may be readying an attack to knock you away again. In fact using a skill while he is doing a knockback attack will keep you from being sent flying! Defeating the beast will net you 5,000 GP and the Black Armor. Plus you'll probably be happy to be done with one of the series laggiest battles. That slowdown...