Following the Observatorium's alarming observation of the dragon star, Aymeric requests that you investigate the corpse of the boss on the wreck of the Agrius in Mor Dhona.
How to Unlock
The Keeper of the Lake dungeon is unlocked by the level 50 Main Scenario Quest The Rising Chorus.
Clear the Agrius Hull
Defeat the enemies in the way to open the path to the first dungeon boss.
Einhander will sometimes attack with Aero Blast, which damages all players.
When Einhander casts Resounding Screech, small untargetable Einhanders will appear, holding ceruleum tanks, which they will drop onto the platform. Keep your distance from the glowing red tanks to avoid the large circle attacks around them. After the first tanks explode, more may begin to glow, so be sure to avoid those as well.
At the same time, Einhander will move to the side of the room and will cast a Quick-firing Cannon attack. The attack will either be a line attack down the center of the room, or a proximity attack from the center of the room. To avoid the attack, go to an edge of the room that Einhander is not facing.
Einhander sometimes casts Heavy Swing, a high-damage attack on the player with highest enmity.
Deactivate the Garlean Barriers
After the battle, go west to the next area. Kill the 6th Cohort enemies and pick up the Imperial Identification Key that they drop. Check on the Magitek Terminal and say Yes to deactivate the barrier.
Go up the stairs and follow the path. Don't attack the Magitek Field Inductor or you will free the Colossus. After you kill the 6th Cohort enemies outside of the barrier, destroy the Magitek Field Inductor, then kill the Colossus. Move on to the next area and do the same. After all the enemies are defeated, pick up the Imperial Identification Key and use it on the Magitek Terminal.
Clear the Ceruleum Spill
Proceed to the next area, where you will find the next dungeon boss.
The gunship will periodically target a player with Flame Thrower, a large cone attack. This attack lasts for several seconds, so get out of the attack marker and stay out of the flame area to avoid damage.
The Gunship will sometimes go to an edge, then create a trail of flame puddles beneath it as it goes across to the other side of the platform. Later in the battle, it will follow this up with another trail of flame in a random direction.
The gunship will periodically target a player with Carpet Bomb, a circle attack.
When a 6th Cohort Signifer appears, kill it quickly, then resume attacks on the gunship.
The gunship may sometimes target a random player with Blitz, which knocks the player back.
Kill the 6th Cohort Vanguard quickly when it appears.
Go south to the next area. Fight your way through the next area while dodging the circle attacks from the Magitek Gunship. Defeat all the enemies in each area to open the path to the next area.
??? Boss Battle
Admonishment is a very wide line attack from the boss.
Inner Turmoil is a circle attack centered on the boss.
Outer Turmoil is a donut attack where the safe area is near the boss.
Phantom Kin summons untargeteable dragons.
Phantom Admonishment is a line attack followed by two smaller line attacks in the direction that the two phantom dragons are facing. Make sure not to stand in front of either of the two dragons.
Disgust damages all players.
Phantom Outer Turmoil is a donut attack, followed by two line attacks in the direction that the two phantom dragons are facing.
The boss will summon two Mirage Dragons, then will begin filling an Aether Gauge. Kill the two dragons before the gauge fills.
Antipathy is an expanding attack, starting as a circle of fire in the center, followed by a ring around the first circle, then a larger ring around that. Stand on the middle ring, then go into the center as soon as the flames have disappeared from the center.
Animadversion does high damage to all players. The damage of this attack is based on how much the Aether Gauge filled before you killed the Mirage Dragons.
Akh Morn places a stack marker on a random player, and hits that player several times. Be sure to stay stacked with that player until the attacks stop.
Condescension is a high-damage attack on the player with the highest enmity.
After the battle, there will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room.
Completing the Quest
Defeating the boss will complete one of the quest objectives for the Main Scenario Quest The Rising Chorus.