The Deacons of the Deep boss fight is like a mix between Prowling Magus and Congregation from Dark Souls II, and the Phalanx fight from Demon’s Souls. When you enter the arena, you’ll see a whole congregation (heh) of enemies gathered in the center, but only one of them is your real target. The real target is the one enemy exuding the red aura.
You’ll want to be careful as you approach the Deacons; the others will surround the red target, and some will attack you with melee, while the others cast spells at you. Thankfully, the extra mob enemies won’t break away from the target and actively chase you, so this will give you lots of opportunities to go in for cheap attacks. These extra enemies don’t have a lot of health, but they’ll continuously respawn as the fight goes on, so make sure to hit the red target as often as you can. Each time you kill the target, the red aura will dissipate and transfer to another random enemy.
After you’ve depleted half of the Deacons’ health bar, stronger and more aggressive enemies will start spawning, and the red targets will become a little tougher as well. But there’s no cause for concern; simply wail on the red targets, and thin out the mobs whenever you have to, and this battle will be over in no time. The Deacons of the Deep isn’t a particularly tough boss fight. Finishing this fight will give you the doll key, which allows you access to the next area of the game.
That’s it for the Deacons of the Deep in Dark Souls III. If you have any additional questions for this fight, or any other Dark Souls III enemy, be sure to leave us a note in the comments so we can help you out, and also check out our ever growing Dark Souls III wiki guide.