To obtain one of the four fae relics, the Fuath require you to go to Dohn Mheg.
How to Unlock
Dohn Mheg, a 4-player dungeon, is unlocked by the level 73 Main Scenario Quest The Key to the Castle.
Whenever the Dohnfast Fuath is tethered to an enemy, try to Interrupt it if you are a tank or ranged physical DPS player, to stop it from casting Watering Wheel, which will give a Damage Up status to the enemy that is tethered to.
Aenc Thon, Lord of the Lingering Gaze
Candy Cane is a high-damage attack on the player highest on the enmity list.
Hydrofall marks one or more random players with large circle attack markers. Move away from the attack markers to avoid this attack.
Laughing Leap will either mark a random location with a circle attack marker or will mark a random player with a stack marker. If there is a stack marker, all players should stand near the marked player to reduce the damage.
Landsblood will create geysers of water throughout the area. These geysers will eventually explode, and more will appear, so you will have to find a safe spot, then look for the next safe spot after the geysers explode, and so on until the geysers disappear. The ring-shaped geysers will NOT explode right away, while the solid circle geysers WILL explode soon. To avoid damage, you should either stand on a ring-shaped geyser, or stand in a space where a geyser has just exploded.
Swinge targets a random player with a large cone attack marker. Move out of the cone attack marker to avoid it.
Fodder will create five Painted Saplings that the boss will tether to. Each player needs to stand between one of the tethers and the boss. Make sure that there aren't multiple players trying to block the same tether.
Tiiimmbeeer damages all players.
The boss will increase in size based on the number of Painted Saplings that it was tethered to. Players who intercepted a tether will also increase in size.
Coiling Ivy traps all players in Painted Roots. Quickly destroy the roots that you are trapped in, then destroy any remaining roots.
On the way to the final boss, be sure to check on the Shell Crown to proceed.
Aenc Thon, Lord of the Lengthsome Gait
Crippling Blow is a high damage attack on the player highest on the enmity list.
Virtuosic Capriccio damages all players and inflicts everyone with Bleeding status. The healer can use Esuna to remove the Bleeding status. However, later in the battle, the boss will cast it multiple times in a row, reapplying the Bleeding status if it was removed.
Imp Choir is a gaze attack. Turn your character to face away from the boss to avoid the attack.
Toad Choir is an unmarked cone attack that will turn players into toads. When you see the boss begin to cast Toad Choir, move behind the boss and you will be safe from the attack.
Funambulist's Fantasia knocks all players back and stuns them. A chasm will form in the area, with a narrow path leading from your side to the boss's side. Don't try to use a dash attack to reach the boss; you will just fall into the pit. (The Trust avatars are the only ones who can successfully dash to the boss.) Instead, carefully walk along the narrow path to reach the boss's side. The boss will be casting Finale. You have to enter the barrier around the boss and quickly attack the Liar's Lyre until it is destroyed. If you do not destroy the Liar's Lyre before the boss finishes casting Finale, the party will be defeated and will have to start the battle from the beginning.
Changeling's Fantasia will transform the boss into a Shade of Fear. Avoid the circle attack markers that it places on random players. It will sometimes turn to face a random player and attack with a continuous, unmarked cone attack. Stay behind the Shade of Fear to avoid this. It will also sometimes raise its tentacles. Avoid being in front of its tentacles when they are raised.
Completing the Quest
Defeating the boss will complete one of the quest objectives for the Main Scenario Quest The Key to the Castle. There will be a cutscene.