Note: you might be able to flag this mission after completing Bat Hunt 5 times after Mission 5-1 is completed.
If you have previously accepted the quest Under Oath, you will be unable to speak to Trion about the mission. You must complete the first portion of Under Oath that involves talking to various people in Southern San d'Oria.
Upon zoning into Castle Zvahl Baileys (consult the map for ease of navigation), move westward to the 4-way intersection at G-8; head south from here, down the stairs that you will soon come to, and then on to the second map through the border of E/F-8.
On Map 2, move south to the bottom of I-9, finding a group of Orcs, then go up the stairs here; move west to G-9 (Yagudo) and go back down the stairs there, being careful to not fall off the circular ledges along the way; turn left from the staircase heading east, when you reach a gate, turn left again. Take the next right turn and make your way to another short stairway appearing on your right at the border of G/H-8 and go up; from there move west to zone into Castle Zvahl Keep.
A short cut is to head to the Goblin section and drop down their hole. Once you climb back up the steps you will be behind the barrier so walk under the arch, hang a right, take an immediate left at the Quadav. Then walk up onto the platform turn left and up the gigantic staircase into the Keep.
Once you enter Castle Zvahl Keep, you will have to open several gates/doors, very often in front of mobs that detect by sight. On Map 1, make your way to either point "A" at H-6 or "B" at H-10 to get to Map 2; on Map 2, strut through a few more doors and gates to "C" at E-8.
On Map 3 you will encounter a series of teleport devices. Be aware that these devices will cancel the Invisible effect after they are used. Also, they will turn on and off from time to time, so you may have to wait on one before it teleports you. Once you use the last device (top-right corner, G-9) you will be on Map 4; now make your way west, and zone to the Throne Room area via a door at G-7.
Finally, go west all the way across the bridge here to the Throne Room door. Examine the door to view a cutscene and, when ready, examine again and select "The Shadow Lord Battle" and enter the BC.
When ready, gather in front of the door. When you enter, a cutscene will happen and the fight begins:
This fight is level capped without a specific level, which means buffs will wear upon entry.
You have 30 minutes to complete this battlefield event.
The Shadow Lord does not attack until you approach, so cast the prefight buffs before getting closer.
In the first phase, he alternates between physical and magic immunity. He has magic immunity until around 50% HP, then starts to switch to physical. From this point he will switch about every 5 minutes or 1000 damage. In this phase he has 9,000 HP.
When he's immune to magical damage (his magic stance), he will mainly cast BLM spells such as elemental debuffs and tier 2 AoE nukes. He also has a weaponskill, Dark Nova (AoE dark damage).
Dark Nova seems to indicate that he will switch to melee stance very soon, after casting a spell or two. So use your TP quickly.
After the first phase is defeated, a brief cutscene will occur as he switches to phase two. He will immediately attack after this cutscene, so there's no time for rest.
In this phase, the Shadow Lord will do nothing but his Implosion attack. This attack hits almost everywhere in the battlefield, but he only has 4000 HP and should go down quickly, however.
Melees should use their two-hour abilities, and Icarus Wing, if possible, to have TP for their best weaponskill. Mages should cast their Curaga spells, and try to keep Stoneskin (and Phalanx, if available) up.
Since hate is mostly irrelevant in this phase, mages should feel free to spam spells.
The battle ends after the second phase. The Shadow Lord gives experience points for a Level 60 mob.
After the Shadow Lord is defeated, another cutscene occurs. Once the cutscene ends, you will be transported back to the entrance of Castle Zvahl Baileys. You will also receive the key item Shadow Fragment.