Regulan bloodworms were a form of living weapons developed by the Regulans. They were uncontrollable and so destroyed their creators.
In 2268, Doctor Jenna Yar and Commander Blodgett had tried experimenting with bloodworms aboard the USS Copernicus. Their aim was to develop a weapon the Federation could use against the Klingon Empire. However, this led to the extermination of the ship's crew. ("Blood and Fire", Parts 1 and 2)