The Ring of the Dead is found in Drangleic Castle, in a room accessed by using the elevator before the Looking Glass Knight boss fight. In order to get the ring, the Milfanito in the cage must be freed by using the Key to the Embedded on the door. When talked to, she will scream and then disappear, leaving the Ring of the Dead as a drop.
The ring makes its wearer appear human regardless of hollowing. The effect is purely cosmetic however, meaning that the ring has no practical use.