Found in the innermost chamber of the Cathedral of the Deep, where the tomb of Aldrich lies. The room is located further from the main praying area, beyond an immense and almost ruined statue concealed in canvas.
The Deacons of the Deep are led by Archdeacon Royce, one of Aldrich's most devoted followers, alongside McDonnell and Klimt. It is mentioned that Klimt attended to Rosaria, whom he deemed as a goddess, and one day lost his faith, but the sequence of events and their causes are not certain, and whether or not he is still alive is difficult to figure.
When Aldrich and McDonnell traveled to Irithyll with a small group of deacons to meet with Pontiff Sulyvahn, Royce and the other deacons stayed to guard their lord's empty coffin from intruders, in the hopes that he would one day return. It is said that McDonnell imparted the spell Deep Soul to Royce and his followers, to aid them in their defense. 
As their name implies, the Deacons of the Deep is a large gathering of Deacons, who mainly rely on pyromancies and close ranged melee attacks to attack the player. They will engage the player in combat, with one of the Deacons glowing with a red aura. Defeating this Deacon will deal damage to the boss's health bar, and cause the light to dissipate and search for another Deacon. Throughout the battle, new Deacons will spawn from Aldrich's coffin as they are defeated.
Once five glowing Deacons have been killed, Archdeacon Royce will reveal himself; Royce can be readily identified by the red glow, as well as the large blue-tipped staff and miter. He will spawn with two big and blue-robed Deacons and four additional heavyset Deacons, who will cast Force when near the player in order to protect Royce. At certain points, they will channel a healing spell to quickly regenerate Royce's health.
- Anri of Astora: After exhausting her dialogue at the Halfway Fortress bonfire at Road of Sacrifices, her summon sign will appear in the prayer area before the Deacons.
- Horace the Hushed: His summon sign can be found near Anri's.
- Sirris of the Sunless Realms: After giving the Dreamchaser's Ashes to the Shrine Handmaid and exhausting Sirris's dialogue when she appears at Firelink Shrine, she will be available to summon.
Damaging or killing individual Deacons does not deal damage to the collective boss health bar, rather this is done by killing the Deacon with the red aura. Each time the target Deacon is defeated, roughly 327 damage is dealt to the health bar and the aura will move to another Deacon. For this reason, it is a viable strategy to try to reduce the health of the individual Deacons without killing them, as the aura may find a weakened one who can then be quickly dispatched. If the player does not kill a glowing Deacon within a certain amount of time, the aura may relocate to a different one.
Be mindful that while large sweeping attacks can hit and kill multiple Deacons at once, this can place the player in unnecessary danger. If possible, try to path around the crowd to attack the glowing Deacon directly, as this avoids the situation of being within melee attack range of multiple Deacons.
Once enough damage has been dealt to the boss health bar, Archdeacon Royce will appear, flanked by two large blue-robed Deacons as well as four red-robed ones. It is highly recommended to kill these two blue Deacons as quickly as possible, as they are considerably stronger than the others, can heal Royce, and will not respawn once killed. From this point onward, only damage dealt to Royce will subtract from the boss health bar.
Once Archdeacon Royce is on the field, the collective Deacons gain a number of Dark attacks, besides retaining their previous abilities. However, these attacks can be interrupted either through attacking the channeling Deacons or Royce himself.
Melee weapons are the best option for defeating the Deacons of the Deep without too much trouble. A halberd or any weapon that performs wide swings will deal great damage to multiple Deacons, allowing players to easily clear a path to the glowing one, however a fast-swinging straight sword gives less opportunity to be staggered by the Deacons' attacks. Another nice option is to use the Hollowslayer Greatsword: at +0 allows you to kill a standard Deacon with two hits and will stagger and let fall the fat Deacons. Spells are not recommended, simply because there are not many area-of-effect spells available to the player at the time of encountering the boss, and the casting time of the spells can easily leave the player vulnerable to being staggered by the Deacons' attacks. Chaos Storm or piercing spells like Soul Spear are effective, though. The pyromancy Boulder Heave is very effective as it deals both physical and area-of-effect damage.
- Fireball - When at range, Deacons will often raise their candlesticks and launch a straight-flying fireball at the player. These attacks deal relatively little damage, but their danger lies in the large number of Deacons who may simultaneously cast it, as well as its stagger, which can potentially combo the player along with other Deacons' melee attacks.
- Force - The fat Deacons, as well as the blue-robed ones that flank Royce, will occasionally cast Force if the player is nearby, in an attempt to stagger them. This attack deals no damage, but can knock the player down and prevent them from approaching the red-glowing Deacon or Royce.
- Curse Fog - A number of Deacons will begin channeling dark energy, and the entire arena will begin to darken, slowly building up the player's Curse resistance bar. Killing or staggering one of the channelers will break the effect, allowing the player's Curse meter to reduce.
- Deep Soul - Royce will raise his staff and shriek, and several other Deacons will also begin channeling dark energy into an orb above Royce. This process takes several seconds, but when completed Royce will flourish his staff and launch a large dark orb at the player; it is very fast-moving and will ignore all obstacles, including pillars and Aldrich's coffin. The orb can deal significant damage, and inflict moderate Curse buildup on the player. Staggering Royce himself will interrupt the process.
- Heal - The blue-robed Deacons that flank Archdeacon Royce can channel a healing spell that will restore a significant portion of his health, about 25% of the entire boss health bar. It can be very difficult to stagger them out of the spell, as players will have very few weapons with poise-breaking ability at their disposal at this point in the game. This ability is best countered by killing the blue Deacons as soon as possible, before they are given the chance to cast the spell.
Soul of the Deacons of the Deep
- Affected by Rapport.
- Affected by Alluring Skulls, with the exception of the deacon covered in a red aura.
- Deacons are weak to Hollowslayer Greatsword.
- Defeating the Deacons of the Deep is required to obtain the Small Doll and, thus, gain access to Irithyll of the Boreal Valley.
- The Archdeacon Set will appear in the boss room after defeating the Deacons of the Deep and reloading the area. It can then be looted from Archdeacon Royce's lifeless body.
- With this, the deacons are one of the only two bosses in Dark Souls III that leave behind any actual remains, besides their leader Aldrich. They are also one of only two bosses with unique dark blue souls, the other also being Aldrich.
- ↑ Excerpt from the Soul of the Deacons of the Deep description.
- ↑ Archdeacon Skirt description
- ↑ Saint Bident description
- ↑ Archdeacon Holy Garb description.
- ↑ Deep Soul description.