The Deacons of the Deep mainly rely on pyromancies and close ranged melee attacks. Halfway through the battle, the room around the deacons may become cursed; if the player steps out of the safe zone, they will die instantly.
When Archdeacon Royce appears, he will spawn with two big and blue robed deacons, which will be constantly casting Force throughout the fight to stagger or even knock the player down and interrupt them from performing actions. At certain points, they will use Great Heal to slowly regenerate the archdeacon's health.
Found in the innermost chamber of the Cathedral of the Deep, where the tomb of Archdeacon Royce lies. The room is located further from the main praying area, beyond an immense and almost ruined statue concealed in canvas.
Once you enter the boss room, you will notice one of the deacons is surrounded by a red mist, focus on him until he dies. The mist will then transfer to another deacon. Every red deacon you kill, deals 426 damage to the boss's health bar. At half health, Archdeacon Royce will arise from the ground, along with two unique blue-robed casters which will protect him with their bodies. Focus all attacks on the archdeacon throughout the rest of the battle.
Occasionally, the deacons will raise their staffs and so will the archdeacon, charging up a big Dark elemental energy ball which will be sent away, homing towards your current position. If you do get hit by it, you will get a slither of Curse damage. The second thing they charge up is a curse fog. Killing one of the casters will stop the channeling and the fog will fade. If you have good armor, decent health and deal decent damage, then the boss battle should be over in a relative small amount of time.
Melee weapons are probably the best option if you want to beat this boss without too much trouble. A halberd or any weapon that performs wide swings will do great damage toward them, even knocking them down if you break their poise.
Magic is not recommended, simply because there are not many area-of-effect spells. 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.
Soul of the Deacons of the Deep
- The deacons are affected by Rapport and some will be affected by throwing Alluring Skulls.
- 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 DS3 that leave behind any actual remains, besides their leader Aldrich. They are also the only two bosses with unique dark blue souls.