Covetous Silver Serpent Ring +1
Fallen foes yield more souls. Received from the old merchant at the Cardinal Tower bonfire after making enough purchases.
Covetous Silver Serpent Ring +2
Fallen foes yield more souls. Received after defeating Mytha, the Baneful Queen in New Game Plus.
Increases curse resistance. Can be purchased from Magerold of Lanafir. He's found by making a right after crossing the Iron Keep bridge and entering the keep proper.
Can also be purchased from Cromwell the Pardoner in the Brightstone Cove Tseldora chapel.
Dark Quartz Ring
Increases dark resistance. Can be purchased from Rosabeth of Melfia in Majula.
Dark Quartz Ring +1
Increases dark resistance. Increases dark defense. After defeating The Duke's Dear Freya, you'll head down a hallway into a study. Take out the enemy in here, and he'll drop this ring.
Dark Quartz Ring +2
Increases dark resistance. Can be dropped by Leydia Witch enemies in Undead Crypt.
Increases invasion chance. Can be purchased from Titchy Gren at the Undead Purgatory bonfire.
Increases magic, lightning, fire, and dark defense. Can be purchased from Royal Sorcerer Navlaan in Aldia's Keep.