Deacons of the Deep Dark Souls III Guide
Last update: 11 May 2016
Remember: equip yourself with armor that provides solid protection against curses. It will be nearly impossible to win if the bar fills up. A curse can be reduced by killing the boss quickly or using a Purging Stone. Those can be purchased from Yuria in Cleansing Chapel, but only if you used Yoel's ability 5 times already (Yoel dies and Yuria replaces him) and you didn't heal Dark Sigil at Fire Keeper. Furthermore, if you have already travelled to Farron Keep and found the proper ashes that will provide progress to Sirris's quest, you will be able to summon her during the battle (just before you enter the fog, on the left side).
The first stage of the battle is a fight against a standard group of opponents. However, the large number of your foes will often make it hard to reach "the right one". The boss will receive damage only if you kill the Red Deacon. Once you slay him, the read aura will jump to another one, and thus make him your new target.
To be more effective at killing enemies, just avoid or dodge most of them. The entire group moves rather slowly so take your time to lure the right one. If you have direct access to the glowing Deacon, rush and immediately kill him. A weapon with a long range that allows to strike multiple foes at once will be extremely useful.
Remember: don't bother with killing every single creature. They will respawn over and over again, so focus on the Deacon that is currently glowing. If you use magic, cast spells that kill several opponents at once.
During the second phase of the battle (when the HP is about 50%) the archdeacon will appear. From now on, he is your main target, and time is against you. He will be followed by more powerful deacons (in blue robes) who are more aggressive. The very archdeacon, however, will not be chasing after you, and will focus on casting spells. If you were smart enough, you should have the right armor and a Purging Stone with you. If the bar is charged in about 50%, you'd better use the item or you will be killed by the curse immediately. On top of that black orbs will come from the fog and deal damage.
You need to be quick and aggressive to survive. Once the archdeacon appears, concentrate on him and disturb the ritual. If you run out of Purging Stones, you won't have much to lose and may as well jump between your foes and slash them as much as you can. Your opponent is vulnerable to melee attacks and his own strikes are not impressive as well. The more the battle will take, the more possible it is that you receive tremendous damage or even die.
Reward: Soul of Deacons of the Deep, Small Doll, 14 000 souls
Location: Cathedral of the Deep