The Brass Heart Shield is a vampiric artifact.
A Tremere vivisectionist created this device in the 1400s. This brass plate, when inserted over the heart of a vampire, deflects all non-magical stakes. The Tremere was murdered by a number of Sabbat who literally ripped him apart. They discovered that the plate could be placed under the skin with a small vertical incision. The plate then sinks through the flesh and rests above the heart. Once inserted within a given vampire, the plate is only removable upon the death of the vampire wearing it.
Players Guide to the Sabbat, p. 153-154