Surprise! He's not here after all.
Dark Souls 3 boss: how to beat Deacons of the Deep
The Deacons of the Deep boss arena is behind the altar in the Cathedral of the Deep. The easiest way to reach it is to start from Rosaria's Bedchamber. Simply sprint out past the maggot men, go downstairs, cross the bridge, follow the path around past the main doors, and take the lift down.
From here, if you're going straight into the battle you can just sprint out and around the altar, and enter the mist before anything can catch you. If you want to hang about outside the door and summon assistance, you'll probably want to kill at least the two priests on patrol and the standing Cathedral Knight; you can avoid aggroing the remaining enemies just by keeping your distance.
Depending on your build, this boss can be very easy. The battle is against a gaggle of priests of various shapes and sizes. They’re weak and attack slowly, but they respawn rapidly - and endlessly.
To damage the boss, you need to find the one priest with a red glow. Cut them down, and the glow will transfer to another body.
What makes this tricky for some players is that the priests tend to huddle together. If you’re using a single hit ranged attack (a soul dart or an arrow), a thrust weapon (spear, rapier) or anything else with a small point of impact, the pack usually blocks the damage intended for your target. If you muscle in close to the target, the pack closes around you.
The trick is to bring a weapon with a wide slashing attack, or area of effect magic. Circle the pack until its natural movement leaves your target more or less exposed, then dive in hacking and slashing (or alternately, begin throwing magic around). Roll away after a few hits, before the priests outside of your immediate range can effectively target you.
In the second phase, the priests are more aggressive, attacking more quickly. More importantly, four more unique priests will appear; they’re larger and wear blue robes. These guys cast a curse spell that gradually fills the room; if it fills completely, you will die. Killing any one of the four halts the spell casting, but they respawn.
If you’ve been inflicting decent damage this isn’t too hard; dive in and hit the target a few times, then go kill one of the four spell casters. Repeat, alternating between the two, until you can easily finish off the boss's health bar before the spell casting ends. Bring the hammer down.
This battle is much easier with two players, of course; NPCs are available to summon outside if you need (although if you are running version 1.01 or below, viewing Sarris's summon sign will crash the game), and it’s a favourite place for Warriors of Sunlight, since they’re confident of an easy victory. Do them a favour; they need the faction rep.
You’ll receive the Soul of the Deacons of the Deep and the Small Doll, which acts as a key to a later area, for your efforts.
If you travel away and back again or rest at the bonfire, you can also loot the Archdeacon's set from a corpse (thanks matthew243342).
Unfortunately, this was all a trick; there’s no Lord of Cinder here. Head back to Firelink Shrine and speak to Anri of Astora to find out where to go next.