Archaeologists in Bulgaria have discovered a medieval ring thought to have been used to to commit multiple political murders.

The bronze ring is more than 600 years old and was found at the excavation site of Cape Kailakra, a place where 14th century Bulgarian aristocrats lived.

More than 30 other pieces of jewelry were also found at the site, including gold rings and pearl earrings but archaeologists say this ring is special.

Drilled into side of the ring is a small cavity, archaeologists say was used to hide poison probably used to murder friends of the aristocrats in the Dobrudja area.

Expertly and exquisitely crafted, the ring is thought to have been imported from Italy or Spain according to dig leader Bonnie Petrunova, deputy director of Bulgaria's National Archaeology Museum.

"I have no doubt that the hole was deliberately set," Petrunova said in a press release. "The hole is made so...the poison can be added at any given moment."

The ring would have been worn on the pinky finger of a man's right hand. The cavity provided an easy way to pour poison into an enemies glass without being detected.

"This explains many of the unexplained deaths among nobles and aristocrats close to Dobrotitsa," the press release reads.

Petronuva believes the discovery of the ring is the oldest proven case of serial murder.