At the end of the Cathedral of the Deep you encounter the boss, the Deacons of the Deep. While you can summon both Anri of Astora and Horace the Hushed before you enter the boss chamber, with proper camera control you won’t really need help from either of them, but if you have an Ember active there’s no harm in having a little assistance. The Deacons of the Deep boss is comprised of a large group of pyromancers all moving in a closed formation. Any of these pyromancers can and will throw a moderately slow-moving fireball toward you character throughout the battle.
You can kill any of the pyromancers with relative ease, but they will be resurrected in a short time. The only way to inflict damage to this boss is to attack the pyromancer with a glowing red head. You can attack this specific pyromancer several times before he goes down, then the red flame will move to another pyromancer (generally in the back of the group) and you’ll need to seek them out and attack again.
Once the Deacons of the Deep health bar hits 50 percent, several larger pyromancers and the head deacon will appear. At this point the flame will no longer move, but the larger pyromancers will be more vigilant in protecting the head deacon. Simply continue to focus on the head deacon and this fight will be over quickly. If you do have help from the NPCs or a friend online, it draws some of the fire blasts from the pyromancers, but the general strategy doesn’t change at all.
When the battle is over, you obtain the Soul of the Deacons of the Deep and a Small Doll. Light the Deacons of the Deep bonfire, then collect the Archdeacon White Crown, Archdeacon Holy Garb and Archdeacon Skirt in front of the large structure in the middle of the room. If you head back to the Firelink Shrine you’ll notice Anri and Horace have appeared up the stairs (opposite the thrones). You can speak with them to further their story line. To progress further into the game you should head to the Farron Keep bonfire if you haven’t done that area already.