Found in the Profaned Capital.
From the Profaned Capital bonfire, climb down the ladder and exit through a hole in the wall. The path eventually leads to a toxic swamp and the ruins of a chapel. Drop into the swamp and head inside a cavern inhabited by several Sewer Centipedes on the opposite side of the chapel. The ring can be found on a corpse within such cavern.
Increases Curse resistance by 90.