Old Dead One Soul
The Old Dead One Soul is a drop from The Rotten in The Black Gulch on a bonfire intensity of 2 or higher, this can be achieved by either going to NG+ or by using a Bonfire Ascetic on the bonfire right before the boss.
The Old Dead One Soul can be traded with Ornifex in the Brightstone Cove for the Crypt Blacksword or with Straid in Lost Bastille for the Numbness hex.