Death's ring is a ring that, as a Horseman, Death wears. Death, in his physical form, seems to need his ring for at least some of his powers. Although, unlike the other Horsemen, the taking of his ring doesn't hurt him at all, differentiating him from the rest. To what extent he can access his powers without the ring is unknown. The ring was destroyed with Death when he crumbled to dust upon Dean killing him with Death's Scythe.