From the Cleansing Chapel, walk through the shortcut door on the left (having opened it previously) and ride the lift nearby. The player will enter a room with a Cathedral Grave Warden wielding a crossbow. Kill it and proceed to the ledge outside, hugging the left wall. Climb up the ladder and kill the lone Deacon of the Deep on the floor above to obtain the ring.
Grants one additional Attunement slot while equipped.