Here you will conclude Anri of Astora's questline. Also, just before entering Irithyll you can be summoned by Sirris of the Sunless Realms and assist her, as she is being attacked by Creighton the Wanderer.
Found on an altar. To get to it, go in front of the fog gate to Pontiff Sulyvahn, turn around and drop down ontop of the altar to get the ring.
Dropped by Sulyvahn's Beast in Water Reserve bonfire. Behind illusory wall.
Found above the Pontiff Sulyvahn bonfire in Irithyll of the Boreal Valley
Found on a corpse on a ledge below a broken railing immediately after the dark room filled with Irithyllian Slaves, shortly before reaching the water.
Found below the pedestal containing the Ring of the Sun's First Born.