Upon zoning into Castle Zvahl Baileys , go straight until you encounter the first intersection and make a left. Go down a small flight of stairs and then turn right and follow the left wall and you will enter the Orc section of the castle.
Go through the area to the west again and eventually you will zone into Castle Zvahl Keep.
Once you enter Castle Zvahl Keep, you will have to open several gates, very often in front of mobs that detect on sight.
You will encounter a series of teleport devices. These devices will cancel the Invisible effect after they are used. Once you use the last device, run to the west exit of the map where you will find a zoneline to the Throne Room.
When ready, gather in front of the door. When you enter, a cutscene will happen and the fight begins. Cast the prefight buffs before getting closer.
This fight is level capped at level 75. (Meaning that buffs will wear upon entry.)
The Shadow Lord has 2 forms. In the first form 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.
When Shadow Lord starts taking 0 damage from physical attacks, mages need to nuke repeatedly and rest when melee are able to deal damage again.
When he's immune to magic, he uses a Dark Nova WS (AOE Darkness damage) and casts Black Mage spells.
Dark Nova seems to indicate that he will switch to physical immunity very soon, after casting a spell or two. So use your TP quickly.
When he's immune to physical, he uses a variety of WS (Giga Slash, Kick Back, Umbra Smash).
After the first form is defeated, a cutscene will appear. The second form will show up after the cutscene and does not wait for you to rest.
This form of Shadow Lord will do nothing but a AOE attack but only has 4000 HP and should go down quickly.
Melees should use their 2 hour abilities and Icarus Wing if possible to have TP for their best weaponskill. Mages should cast their Curaga spells.