A primal has been summoned in Castrum Fluminis and it is up to you to defeat it.
How to Unlock
Castrum Fluminis, an eight-player trial, is unlocked by the level 70 Main Scenario Quest The Primary Agreement.
Torment Unto Death is an unmarked cone attack at the player highest on the enmity list, who should turn the boss to face away from the party. Other players should always avoid being in front of the boss.
Zashiki-Asobi will cause fans to float above the area. This will start with a fan that appears in the center, followed by fans that appear in a ring around the edge. These fans will each place a large circle attack marker beneath them, in the order that they appeared. Players should stand near the fan at the edge that does not have another fan next to it, and move away from the circle attack markers that appear. Then when the center attack marker disappears, players should move into the center.
Steel of the Underworld is a cone attack at the player highest on the enmity list. Other players should move away from the cone attack marker to avoid the attack.
Reprimand damages all players.
Midnight Haze creates two Midnight Haze enemies. Players need to kill them before they collide with each other and explode.
Lead of the Underworld places a line stack marker on a random player. All players should stand in the line stack marker to reduce the damage of the attack.
After a while, the boss will cast Nightbloom, which damages and stuns all players and indicates that the battle has entered the second phase.
During this phase, defeat the tethered enemies as quickly as possible to try to minimize the Suffering gauge that builds up throughout this phase.
The Spectre of Zenos will stun and knock all players back, and will repeatedly attack with multi-target attacks.
After the Spectre of Zenos is destroyed, the boss will attack with Nightbloom, damaging all players. The higher the Suffering gauge at the time this attack, the more damage the attack will do.
Selenomancy will divide the area into light and dark. The longer you stand on a color, the more stacks of Moonshadowed or Moonlit you will obtain. You will die if you get five stacks of either status, so be sure to move to the other half of the area to switch which status you have and reset its stack counter to 1.
Two orbs will appear, and they will each do a donut attack. Stay out of both of the donut attack markers to avoid damage from them.
Antitwilight and Perilune damage all players and remove the black and white halves of the area.
Dance of the Dead does high damage to all players and indicates that the battle is entering the final phase.
Lunacy targets a random player with a stack marker. All players should stack together with the marked player and stay together until the continuous attack stops.
Dark Blade and Bright Blade are large cone attacks that cover more than half of the area. Move out of the attack marker to avoid the attack.
Completing the Quest
Defeating Tsukuyomi will complete one of the quest objectives for the Main Scenario Quest The Primary Agreement. 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.