Death rings were once one of the oldest items in Tibia. It was replaced in an update by Stealth Rings at the same time the other magical rings were introduced. It reduces your shielding by 10 points and has energy for 8 minutes. Quite valued on the market nowadays since they can be used as quest items and usefull in wars because of its death protection. Using a death ring when your shield skill is originally 10, it is reduced down to 0. It's also worth mentioning that even though shield is reduced by 10 levels, your shielding's percents won't go up faster.
After Winter Update 2007 the value of the ring decreased, because now it's obtainable from a quest. The attributes are only shown when worn.