Castrum Fluminis Trial: How to Beat in FFXIV

Castrum Fluminis is a Final Fantasy XIV (FFXIV) trial, or raid, designed for an alliance of 8 players, and is unlocked by the level 70 Main Scenario Quest: The Primary Agreement. In it, players will take on Yotsuyu goe Brutus, who has fused with the Primal Tsukoyomi itself. To say that the encounter will be hard honestly feels like an understatement. The battle in Castrum Fluminis is one of the best boss fights in FFXIV, and perhaps one of its toughest as well.

Much like every raid boss in FFXIV, the encounter within Castrum Fluminis demands coordination, strategy, and skill from each participant. Players must face a variety of mechanics, including complex boss patterns, positional attacks, AoE (Area of Effect) damage, and interactive environmental hazards. Communication and teamwork are key to overcoming these challenges successfully.

Completion of Castrum Fluminis rewards players with valuable items, including gear, weapons, crafting materials, and tokens. These rewards can be used to enhance character progression, aesthetics, or sold for in-game currency. Additionally, the satisfaction of conquering such a formidable trial and the narrative progression it unlocks are among the most fulfilling rewards for players.

This article delves into the depths of the Castrum Fluminis trial, uncovering its mechanics, the thrill it offers to players, and most especially, how to beat it.

How to Beat the Castrum Fluminis Trial – Phase 1

The epic battle begins with Tsukyomi alone. Immediately, she will cast Reprimand, which is a moderate raid-wide damage attack. Following that, she will cast Nightfall two times. This attack can have one of two effects, depending on which weapon she currently wields when she uses it and what Yotsuyu will say while she’s casting. 

The first one, “gun” Nightfall, will force players to stack to split heavy damage. Meanwhile, the other one, “spear” Nightfall, is a straightforward cone attack that targets one tank, one DPS, and one healer. To avoid overlapping the spear damage, we highly recommend that you assign sides of Tsukoyomi for each role.

After Reprimand and Nightfall, Tsukuyomi should also attack players with the Nightbloom. This is a heavy raid-wide damage that inflicts Down for the Count, which also serves as a phase transition. Take note that the phase transition lasts for 16 seconds only, so use this time to refresh over-time effects on the boss before she finishes casting.

How to Beat the Castrum Fluminis Trial – Phase 2

Heading into phase 2, Tsukoyomi should fade from the battlefield, which will now be inside of Yotsuyu’s memories itself. And this time, you’d be fighting Yotsuyu. Throughout this phase, Yotsuyu will incur Suffering, and once she reaches 100 Suffering stacks, the next Nightbloom cast will automatically wipe out the entire party, forcing you to restart the whole encounter. However, the Suffering should not affect the fight itself as long as it does not reach 100. With that in mind, it’s best you throw out your greatest attacks before it reaches just that.

A Specter of the Matriarch and a Specter of the Patriarch should spawn to fight alongside Yotsuyu in this phase. On top of that, more Specters of the Homeland will join the fight as well. Thankfully, they aren’t that tough to defeat, they are there mostly just to annoy you and force your focus out of the Suffering countdown. You’d have to deal with multiple Specter waves in this phase.

On the final wave of Specters, Specter of Zenos, will cast Concentrativity on the raid party. This deals moderate raid-wide damage and yet another Down for the Count infliction. While the debuff wears off, the Specter is met by a Specter of Gosetsu who would attempt to hold him off. All that aside, the party should focus on slaying the Specter of Zenos as quickly as they can. The particular Specter repeatedly casts Dispersivity, which does moderate raid-wide damage while also increasing Yotsuyu’s Suffering by about 6 points every time.

How to Beat the Castrum Fluminis Trial – Phase 3 Part 1

After beating the Specter of Zenos, Tsukuyomi will be reborn, signifying the start of phase 3. Right from the start, she will cast Nightbloomm. This again transforms the battlefield. This time, the battle will take place in Tsukuyomi’s domain, and it’s nothing short of majestic. That said, do not be distracted by the gorgeous visuals and just focus on the battle itself. Following the change of scenery, Tsukuyomi will cast Supreme Selenomancy. This is one of the main mechanics of the raid’s third phase, turning all of her attacks into magic.

Additionally, Supreme Selenomancy also inflicts each party member with four stacks of either Moonshadowed or Moonlit. While the debuffs cancel each other out, they can inflict Doom and Bleed if allowed to reach five stacks. Supreme Selenomancy also divides the whole platform you are currently on into two. The other half is white, while the other is dark. This represents the quarter-moon. Every five seconds, anyone standing in the dark half of the platform will gain a stack of Moonshadowed. Similarly, the ones stuck in the white half should gain an equal amount of stack of Moonlit. And if that isn’t enough yet, there will also be two orbs on one side of the moon. This will pulse repeatedly in an interval which you should try your best to avoid.

How to Beat the Castrum Fluminis Trial – Phase 3 Part 2

After pulsing, the moon will wane or wax, covering all but a crescent of the platform with light or darkness. Following this, the boss will cast Nightfall again followed up by a brand new attack, the Moonfall. This marks a tank, a healer, and a DPS for a meteor that will target where they are currently standing when the marker fades for a devastating AoE attack that leaves behind a large meteor. Take note that the meteor will adopt the color of the platform they fall into, and at least one meteor must be dropped into the fading color.

This is because, after Moonfall, Tsukuyomi will cast Midnight Raid, shadowing the platform with the dominant color, except for the stuck meteors. This means that the only way to survive Moonshadowed or Moonlit in this phase is to go to the meteor of the opposite color of Midnight Rain. That said, the party cannot move there immediately. They must first wait for the AoE pulses of the orbs and the meteors before they could. Which is confusing, to say the least.

After this, Tsukuyomi will cast Perilume or Antitwilight. Both attacks can instantly kill anyone not in the meteor of the correct color. Meanwhile, those that are safely standing on the correct meteor will receive only light damage. This will remove the moon colors. Following this, she will cast Reprimand again followed by Zashiki-asobi. This spawns fans all over the platform which will eventually explode, dealing significant damage. She then casts a random Nightfall as soon as the fans start exploding, followed by yet another massive attack. Following that, the whole phase 3 mechanic should restart.

How to Beat the Castrum Fluminis Trial – Phase 4

After repeating the whole phase 3 mechanic, Tsukuyomi will cast Dance of the Dead, dealing major raid-wide damage. Moreover, this adds an extra effect to her attacks Bright Blade and Dark Blade. For example, if she casts a Blade spell with Full Moon, anyone close to her hitbox will take massive damage. Otherwise, those that aren’t close to her hitbox will incur the same damage. 

Basically, Tsukuyomi will just add variations to her attacks from the previous phases in addition to this new mechanic. This may be very overwhelming to handle for some but you can quickly get used to it after a few tries. Just stay focused and always pay attention to your own health and the party. Eventually, the Castrum Fluminis Trial should end, with your party as the victor. The guys at Square Enix really outdid themselves with this raid boss considering how punishing yet fun it is to beat.